Answer
1200 m/s.
Work Step by Step
The wavelength is found from the relation for the principal maxima for a diffraction grating, equation 24–4.
$$d sin \theta = n \lambda$$
$$\lambda = \frac{d sin \theta}{n}$$
Find the velocity of electrons that have the same wavelength. Assume they are nonrelativistic.
$$\frac{d sin \theta}{n}=\lambda = \frac{h}{p}=\frac{h}{mv}$$
$$v=\frac{hn}{md sin \theta}$$
$$=\frac{(6.626\times10^{-34}J \cdot s)(1)}{(9.11\times 10^{-31}kg)(0.000010m)(sin 3.6^{\circ})}\approx 1200 m/s$$
