You are given the perimeter of a rectangle of 20 inches. You are also given the length of the rectangle in 3 inches. Note that the perimeter of a rectangle is 2L + 2W where L represents the length and W the width. Also, the area of the rectangle is length times width.
P = 2L + 2W
20 inches = 2(3 inches) + 2W
2W = 20 – 6
2W = 14
W = 7 inches
A = L*W
A = (3 inches)(7 inches)
A = 21 square inches