In this question the sale price and the original price of the shoe purchased is given. We need to find the amount of discount Jessica has received for purchasing shoes on sale.
If the width is 2p then the width of both sides is 4p. Subtract that from the total perimeter and you get 16p-4p= 12p. Divide that by two because there are 2 side lengths. So, one side length is 6p.