What is the wavelength of an electron?
The mass of electron is never given in question you should remember it. Mass of electron is $9.109 \times 10^{-31} \mathrm{kg}$.

Now read the question Kinetic Energy is given, so from that you can easily find velocity of electron.

$$K.E.=\frac{1}{2}mv^2$$

so You will get $v= 6 \times10^6 \mathrm{m/s}$

Now you can substitute value of m and v in formula $\lambda = \frac{h}{mv}$

You will get wave length of electron as $0.12 \times 10^{-9} \mathrm{m}= 0.12 \mathrm{nm}$