Answer
0.232 nm.
Work Step by Step
The maxima occur at angles $\theta$ given by $2d sin \theta=m \lambda$, where d is the spacing between adjacent atomic planes. “Second order” means that m = 2.
$$d=\frac{ m \lambda }{2 sin \theta }=\frac{ 2(0.0850\times10^{-9}m)}{2 sin 21.5^{\circ}}=2.32\times10^{-10}m$$
