Answer
$a)0^0$
$b)90^0$
Work Step by Step
If two angles are Complement then sum of these angles is $90^0$
If two angles are Supplement then sum of these angles is $180^0$
$a)\alpha+90^0=90^0\Rightarrow \alpha =90^{0}-90^{0}=0^{0};$
$b)\theta+90^0=180^0\Rightarrow \theta=180^{0}-0^{0}=90^{0}$