Calculate the wavelength associated with a neutron having a mass of 1.675 * 10^-24 g and a kinetic energy of 6.21 * 10^-21 J.

Find the speed of the neutron using:

E_k = \frac12 mv^2

Then use the equation of Max Planck:

E = \frac{hv}{\lambda}

to get the value of lambda, the wavelength.