Electrons moving in a beam have the same de Broglie wavelength as protons moving in a separate beam. The proton beam moves at a speed of 3.1 x 10^4 m/s. What is the speed of the electrons?

From the data sheet, the de Broglie wavelength, 𝜆 = h/p = h/(mv)
The electron wavelength equals the proton wavelength so: 𝜆_e = 𝜆_p
So: h/(m_ev_e) = h/(m_pv_p)
m_ev_e = m_pv_p
so the electron speed, v_e = (m_pv_p)/m_e
From the data sheet, the rest mass of an electron and a proton respectively are: m_e = 9.11x10^-31kg, and: m_p = 1.67x10^-27kg
Subbing these values along with the proton speed from the question gives:
v_e = (1.67x10^-27 x 3.1x10⁴)/( 9.11x10^-31)
v_e = 5.7x10⁷ms^-1