Question

If the wavelength if determined to be 0.018 nm, at what velocity is the electron traveling?
(The mass of an electron is 9.11x10^-31kg)

I know the answer is 4.0x10^7 m/s, but i cannot figure how they got this. Could someone please explain it to me and show me?
Thanks!!
wavelength = h/mv
0.018E-9 m = 6.626E-34/9.11E-31*v
v = (0.018E-9*9.11E-31)/6.626E-34 = 4.04E7 m/s which rounds to 4.0E7 m/s to two significant figures (2 s.f. allowed by the 0.018 nm)

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