Perimeter of a rectangle formula:
P = 2L + 2W
Length equals:
L = 2W + 15
Substitutute 2W + 15 for L
78 = 2(2(15 + W) + 2W
Multiply the inner parentheses
78 = 2(30 + 2W) + 2W
Multiply the parentheses
78 = 60 + 4W + 2W
Add like terms
78 = 60 + 6W
Subtract 60 from both sides
18 = 6W
Divide both sides by 6
3 = W
Length equals:
2W + 15 = L
Substitute 3 for W
2(3) + 15 =L
Multiply the parentheses
6 + 15 = L
Add like terms
21 = L
