The perimeter of a rectangle equals the sum of the lengths of its four sides. If the width of a rectangle is 3 inches and its perimeter is 42 inches, find the length of the rectangle

P = 2L + 2W

42 = 2L + 2(3)

42 = 2L + 6

42 - 6 = 2L

36 = 2L

18 = L