Answer
a) $\lambda=0.13m$
b) holes are smaller than photon wavelength
Work Step by Step
(a) We know that
$p=\frac{h}{\lambda}$
This can be rearranged as:
$\lambda=\frac{h}{p}$
We plug in the known values to obtain:
$\lambda=\frac{6.63\times 10^{-34}}{5.1\times 10^{-33}}$
$\lambda=0.13m$
(b) We know that the holes on the metal screen of the oven are very small compared to the wavelength of photons.