Answer
$5$ and $3$.
Work Step by Step
Our equations according to the exercise are: $2x+2y=16, xy=15$.
So $x=8-y$, thus $y(y-8)=15\\y^2-8y-15=0\\(y-3)(y-5)=0$
Thus $y=3,x=5$ or $y=5,x=3$, but these are the same, so the dimensions are $5$ and $3$.