need help please?

Question

tried getting it but got it wrong. have one submission left.

Calculate the wavelength, in nanometers, associated with an electron traveling 90.0% of the speed of light.
_______ nm

Answer 1

Wave length = h/p but p is not given by rest massx.9c but
rest {mass/sqrt[1-(.9c/c)^2]}.9c = (9.1x10^-31/sqrt[1-.81])x.9x 3x10^8 = 10.8x10^-23 kg ms^-1
lambda = 6.6x10^-34/10.8x10^-23 m= 0.61x10^-11 = 61 nm