Problem with forming a context-free grammar describing a language
$S \to aSb \mid Abb, A \to aaa \mid aab \mid \ldots \mid bbb.$ That's it.
First create an equal number of $a$s and $b$s with $S$. Eventually go to the $Abb$ rule creating two extra $b$s and then terminate with $A$ to any string of length $3$.