To what speed must an electron be accelerated for it to have a wavelenght of 3.0cm?

Answer 1

The "de Broglie wavelength" of an object with mass m and velocity v is

lambda = h/(m v)

where h is Planck's constant and m is the mass. This assumes that v is much less than c; otherwise use the relativistc momentum in the denominator.

Rearrange that equation and solve for velocity. I assume you know the values for h and the elctron mass.

Answer 2

To what speed must an electron be accelerated for it to have a de Broglic
wavelength of 5.0 cm