Having trouble with this seemingly simple problem, help is much appreciated!

Three sides of a fence and an existing wall form a rectangular enclosure. The total length of fencing used for the three sides is 240 ft. Let x be the length of each of the two sides perpendicular to the wall.
a. Draw a picture and write a formula for the area of the enclosure.
b. Then find the value for x that would give an area of 5500 ft^2

a. Pictures easy but would the formula for the area of the rectangle be x times y = 5,500?

b. Don't really know how to find the exact value for x, so I just used trial and error to find x to be around 214.35 and y to be around 25.65.

Help with finding the exact value and the formula would be great.