L2 norm of homogeneous harmonic polynomial

Question

suppose that p is a homogeneous harmonic polynomial of k-order,i.e.$$\Delta p=0 \qquad in \mathbb{R}^{d} $$ my question is how to prove the following inequality: $$||p||_{L^{2}(B_{2r})} \leq C(d)2^{k} ||p||_{L^{2}(B_{r})} $$ for all $r \geq 1$ .