Find a CFG for $a^{2n}b^na^k$ where $n,k \ge 0$

Question

I need to provide a CFG for $L=\{a^{2n}\cdot b^n \cdot a^k | n,k \ge 0\}$ .So I am perplexed on how to account for $a^k$ as $S -> aaSb | \epsilon$ gives me $a^{2n}b^n$ but if I do anything like $S -> aaSbT$ the I start getting garbage like, $aaaaSbTbT$.