P = 2(L + W) <== this is perimeter of rectangle formula
P = 50 ...perimeter = 50
L = 3W + 1....length is 1 more then 3 times its width
now we sub
50 = 2(3W + 1 + W)...simplify
50 = 2(4W + 1)...distribute thru the parenthesis
50 = 8W + 2...subtract 2 from both sides
50 - 2 = 8W
48 = 8W...divide both sides by 8
48/8 = W
6 = W <=== width is 6 inches
L = 3W + 1
L = 3(6) + 1
L = 18 + 1
L = 19 <=== length is 19 inches
