Let $u: \mathbb{R}^n \rightarrow \mathbb{R}$ be a solution of the elliptic PDE
$$ \sum_{i,j}^n a_{ij} D_i D_j u = 0,$$
where the $a_{ij} $ are constants.
Then we know by regularity theory (e.g. Evans Chapter 6) that $u \in C^\infty$.
But I also read that we have the following estimate:
$$ \sup_{B_{R/2}}|D^k u(x)| \leq \frac{C}{R^k} \sup_{B_R}|u(x)|.$$
How do we get this last estimate?