How to find perimeter of something?

Question

A rectangular swimming pool has a width that is 6 feet less than its length. The perimeter of the pool is 84 feet. What is length of the pool?

Answer 1

l=length

l-6=width

perimeter=2*length+2*width

84=2*(l)+2*(l-6)

84=2l+2l-12

84=4l-12

96=4l

l=96/4=24 feet