Like this (I only have to fix all the n+1's, they should be gammas): $$\begin{aligned}
& \|\mathcal{F}f \|_{\alpha, \beta} = \sup_x \left | x^\alpha D^\beta \mathcal{F}f(x) \right | = \sup_x \left | (-2 \pi i)^\beta \mathcal{F} x^\beta D^\alpha f(x) \right | \leq \\
& \max(K, K^\prime) \left ( \| x^\beta D^\alpha f(x)\|_\infty + \| x^{n+1} x^\beta D^\alpha f(x) \|_\infty \right ) < \infty
\end{aligned}$$