Speed of light = 3x10^8 m/s
Mass of electron = 9.11x10^-31 kg
70% of 3x10^8 = 2.1x10^8 m/s
206.8*9.11x10^-31 = 1.88x10^-28 kg
Equation for wavelength is: wavelength= plancks constant / (mass*velocity)
Plancks constant = 6.62x10^-34 m^2 kg/s
Plug all of these in and you get: 6.62x10^-34 / (1.88x10^-28/2.1x10^8) = 739.47
Your answer is 739.47 m
