Please help me with de Broglie waves problem.

Question

Planck's constant is 6.6 x 10^ -34 (J)(s) and the proton mass is 1.7 x 10^ -27 kg. The speed of a proton whose de Broglie wavelength is 4.0 x 10^7 m is:

A. 7.8 x 10^ -20 m/s

B. 9.9 x 10^2 m/s

C. 9.7 x 10^15 m/s

D. 3.9 x 10^6 m/s

Answer 1

without relativity

λ = h/mv

so rearrange for v

v = h/mλ = 9.7 x 10^-15 m/s

notice the negative sign makes it different from answer C you gave

.,.,.