$[20$ Points$]$ Find a regular expression for each of the following languages.

$($a$)\{b{a}^n{b}^m$:$n3,m2\}.$

$($b$)\{\begin{array}{ccc}\text{w}\text{i}\text{n}& \{a,& b{\}}^{*}\end{array}$ : every maximal substring of $w$ consisting entirely of symbols $a$

is of length exactly $3$