Answer
$6.08\times 10^{-7}m$
Work Step by Step
We can determine the required wavelength as follows:
$\lambda=\frac{h}{m_ev_e}$
We plug in the known values to obtain:
$\lambda=\frac{6.63\times 10^{-34}J.s}{(9.11\times 10^{-31}Kg)(1200m/s)}$
$\lambda=6.08\times 10^{-7}m$
