Answer
$-3$
Work Step by Step
$3$ cis $180^{\circ}$=$3(\cos180^{\circ}+i\sin180^{\circ})$
It is known that $\cos180^{\circ}=-1$ and $\sin180^{\circ}=0$
Substituting these values in the expression and solving:
$3(\cos180^{\circ}+i\sin180^{\circ})=3(-1+i(0))=3(-1)=-3$
