Here the neutron is in thermal equilibrium with matter and it have a kinetic due which depends upon temperature of surrounding

We have a relation to find kinetic energy of a atomic particle at any Temperature which is

Where E_K is the kinetic energy of particle at temperature T and K is Boltzmann constant

K = 1.38 × 10^-23 JK^-1

Here temperature is

T = 300 K

So linetic energy of neutron is

So kinetic energy of neutron is

E_k = 6.21 × 10^-21J

And we know Kinetic energy of a particle is given by the relation

Where vis the speed of the particle and m is the mass of the particle

Multiplying both side sides by m

Or

So we get

We know de-Broglie wavelength of a particle is given by relation

where