#### Answer

$\frac{5}{2}-\frac{5\sqrt 3}{2}i$

#### Work Step by Step

$5$ cis $300^{\circ}$=$5(\cos300^{\circ}+i\sin300^{\circ})$
It is known that $\cos300^{\circ}=\frac{1}{2}$ and $\sin300^{\circ}=-\frac{\sqrt 3}{2}$
Substituting these values in the expression and solving:
$5(\cos300^{\circ}+i\sin300^{\circ})=5(\frac{1}{2}-\frac{\sqrt 3}{2}i)=\frac{5}{2}-\frac{5\sqrt 3}{2}i$