Answer:
2 m = E / c^2 where m is mass of electron
E = h v where v is the frequency ( nu) of the incident photon
E = h c / y where y is the incident wavelength (lambda)
2 m = h / (c y)
y = h / (2 m c) wavelength required
y = 6.62 * 10E-34 / (2 * 9.1 * 10E-31 * 3 * 10E8) m
y = 3.31 / 27.3 E-11 m
y = 1.21 E -12 m = .0121 Angstrom units