Serway Problem $27\mathrm{.}29-$ the de Broglie wavelength:

a$.$ If the wavelength of an electron is $5\mathrm{.}00{10}^{-7}m,$ how fast is it moving?

Answer:

$1\mathrm{.}46k\frac{m}{s}$

b$.$ If the electron has a speed equal to $1\mathrm{.}00{10}^7\frac{m}{s},$ what is its wavelength?

Answer:

$7\mathrm{.}28{10}^{-11}m$