Wavelength (λ)of electron is given by
λ=h√2mqV
Here
h is planck's constant
m is mass of electron
q is harge on electron
V is potential difference
Putting the values:
λ=6.63×10−34√2×9.1×10−31×1.6×10−19×40×103
=6.63×10−34√116.48×10−46=6.144×10−12=6×10−12
x=6