Question

Can some plz explain how to do this?
Follow the directions to find the missing side lengths in each problem.
1.) A gardener is planning to plant a rectangular vegetable garden during the spring. The
rectangle below represents a plan for the garden with a width of x feet.

a. Find the length (l) of the rectangle if the area of the rectangle is x^2+12x (hint: factor the trinomial x^2+12x+0 using the guess and check method)