IMU calculating velocity

No, which is why I asked.

Which "MPU9250.h" ?

int j;
constant = 0;
for (j = 0; j<100; j++) { 
    if (j>=0){            
      ACCX = IMU.getAccelX_mss();
      //HH = bmp.readAltitude(SEALEVELPRESSURE_HPA);
      constant = (constant + ACCX);
        }
      }
    constant = constant/(j);

This is crazy - why the "if (j >= 0)" ?
What is the value of j at the end of the loop?
Why the expectation that 16 decimal places is achievable, or desirable?