Answer
$a)$ $-3-2i$
$b)$ $2+8i$
Work Step by Step
$a)$ $(3-6i)-(6-4i)$
Evaluate the subtraction:
$(3-6i)-(6-4i)=(3-6)+(-6+4)i=-3-2i$
$b)$ $4i(2-\frac{1}{2}i)$
Evaluate the product:
$4i\Big(2-\dfrac{1}{2}i\Big)=8i-2i^{2}=...$
Substitute $i^{2}$ by $-1$ and simplify if possible:
$...=8i-2(-1)=2+8i$
