$3\mathrm{.}6$ Simplify the following Boolean expressions, using four$-$variable K$-$maps:

$($a$)*,$A'B'C'D$+$AC'D$+BCD+$A'BCD$+BC\text{'}D$

$($b$){?}^{**},x\text{'}z+w\text{'}xy\text{'}+w(x\text{'}y+xy\text{'})$