include the mbed library with this snippet

#include "mbed.h"
#include "LSM9DS1.h"
#define PI 3.14159
#define DECLINATION -4.94 // Declination (degrees) in Atlanta,GA.
#include "motordriver.h"
#include "MAF.h"

//Debugging
Serial pc(USBTX, USBRX);

//For data
//LSM9DS1 IMU(p9, p10, 0x6B, 0x1E);
LSM9DS1 IMU(p28, p27, 0xD6, 0x3C);

Motor lmotor(p22, p19, p20, 0);
Motor rmotor(p21, p18, p17, 0);

int count = 0;

//data
float roll;
MAF avg;

PwmOut speaker(p23);

float printAttitude(float ax, float ay, float az, float mx, float my, float mz)
{
    float roll = atan2(ay, az);
    float pitch = atan2(-ax, sqrt(ay * ay + az * az));
// touchy trig stuff to use arctan to get compass heading (scale is 0..360)
    mx = -mx;
    float heading;
    if (my == 0.0)
        heading = (mx < 0.0) ? 180.0 : 0.0;
    else
        heading = atan2(mx, my)*360.0/(2.0*PI);
    //pc.printf("heading atan=%f \n\r",heading);
    heading -= DECLINATION; //correct for geo location
    if(heading>180.0) heading = heading - 360.0;
    else if(heading<-180.0) heading = 360.0 + heading;
    else if(heading<0.0) heading = 360.0  + heading;


    // Convert everything from radians to degrees:
    //heading *= 180.0 / PI;
    pitch *= 180.0 / PI;
    roll  *= 180.0 / PI;

    if (count > 10) {
        roll = avg.update(roll);
    } else {
        count++;
        avg.update(roll);    
    }

    pc.printf("Pitch: %f,    Roll: %f degrees\n\r",pitch,roll);
    pc.printf("Magnetic Heading: %f degrees\n\r",heading);
    
    return roll;
}

int main() {
    //Calibrate IMU
    IMU.begin();
    if (!IMU.begin()) {
        pc.printf("Failed to communicate with LSM9DS1.\n");
    }
    IMU.calibrate(1);
    IMU.calibrateMag(0);
        
    speaker.period(1.0/200.0); // 500hz period

    while(!IMU.magAvailable(X_AXIS));
    IMU.readMag();
    while(!IMU.accelAvailable());
    IMU.readAccel();
    while(!IMU.gyroAvailable());
    IMU.readGyro();
    roll = printAttitude(IMU.calcAccel(IMU.ax), IMU.calcAccel(IMU.ay), IMU.calcAccel(IMU.az), IMU.calcMag(IMU.mx), 
            IMU.calcMag(IMU.my), IMU.calcMag(IMU.mz));
    float prevRoll = roll;
    float pGain = 1.0/15.0;
    float dGain = 1.0/10.0;
    while(1) {
        while(!IMU.magAvailable(X_AXIS));
        IMU.readMag();
        while(!IMU.accelAvailable());
        IMU.readAccel();
        while(!IMU.gyroAvailable());
        IMU.readGyro();
        /*pc.printf("        X axis    Y axis    Z axis\n\r");
        pc.printf("gyro:  %9f %9f %9f in deg/s\n\r", IMU.calcGyro(IMU.gx), IMU.calcGyro(IMU.gy), IMU.calcGyro(IMU.gz));
        pc.printf("accel: %9f %9f %9f in Gs\n\r", IMU.calcAccel(IMU.ax), IMU.calcAccel(IMU.ay), IMU.calcAccel(IMU.az));*/
        roll = printAttitude(IMU.calcAccel(IMU.ax), IMU.calcAccel(IMU.ay), IMU.calcAccel(IMU.az), IMU.calcMag(IMU.mx), 
            IMU.calcMag(IMU.my), IMU.calcMag(IMU.mz));
        
        /*
        For differential control create another variable that stores the previous 
        roll value. Subtract it from the current to get the difference. Currently, speed
        is equal to pgain*roll+dgain*(roll difference)*/
        float lmotorSpeed = roll*pGain + (roll - prevRoll)*dGain;
        float rmotorSpeed = roll*pGain + (roll - prevRoll)*dGain;

        //adding extra power close to the balancing point to overcome friction.
        //without this the robot isn't moving fast enough to cover the last tiny distance to balance
        if (fabs(lmotorSpeed) < .1) lmotorSpeed*=1.2;
        if (fabs(rmotorSpeed) < .1) rmotorSpeed*=1.2;

        //set speaker loudness and motor speeds
        speaker = fabs(lmotorSpeed/20.0);
        lmotor.speed(lmotorSpeed);
        rmotor.speed(rmotorSpeed);
    }
}