Chapter 2.1, Problem 82E

Problems $81\text{-}84$ refer to the area $A$ and perimeter $P$ of a rectangle with length $l$ and width $w$ (see the figure).

The area of a rectangle is $81\text{ sq in}\text{.}$ Express the perimeter $P\left(l\right)$ as a function of the length $l$, and state the domain of this function.