The wavelength is equal to Planck's constant divided by the ball's momentum.
This means that:
lambda = h / p .............> equation I
Momentum = mass * velocity ............> equation II
Substitute by equation II in equation I, you get:
lambda = h / mv
Now, we are given:
lambda = 8.92 * 10^-34 m
Planck's constant = 6.625 * 10^-34
velocity = 40 m/sec
Substitute with these givens in the above equation to calculate the mass as follows:
8.92*10^-34 = (6.625*10^-34) / (40*m)
mass = 0.0185678 kg
