## Elementary Technical Mathematics

$$l=\frac{P}{2}-w.$$
We are given: $P=2(l+w)$ To find $l$, we divide both sides by $2$ as follows: $$\frac{P}{2}=\frac{2(l+w)}{2}\Longrightarrow \frac{P}{2}=l+w\Longrightarrow l=\frac{P}{2}-w.$$