Answer
4.36 km/s.
Work Step by Step
$\lambda=\frac{h}{p}=\frac{h}{mv}$
The de Broglie wavelength is the same for both particles.
$$\frac{h}{m_ev_e}=\frac{h}{m_pv_p}$$
$$v_p=v_e\frac{m_e}{m_p }$$
$$v_p=(8.00\times10^6m/s)\frac{9.11\times10^{-31}kg}{1.67\times10^{-27}kg}$$
$$v_p=4.36\times10^{3}m/s$$
The proton and electron have the same de Broglie wavelength, and therefore the same momentum. The proton is slower because the proton’s mass is so much more than that of the electron.
