Answer
$$w= 19.5.$$
Work Step by Step
We have:
$P=2(l+w)$
First, we find $w$ by dividing both sides by $2$ as follows:
$$\frac{P}{2}=\frac{2(l+w)}{2}\Longrightarrow \frac{P}{2}=l+w \Longrightarrow w=\frac{P}{2}-l.$$
Then, by substitution, we have $$w=\frac{112.8}{2}-36.9=19.5.$$
