4 because 6(4) = 24 and 24-3=21
Perimeter of a square: side length * 4, since squares have equal side lengths
Take 2x - 3 for the side length and plug it into the formula.
(2x - 3) * 4 can be rewritten as 4(2x - 3)
Distribute 4 inside the parentheses.
8x - 12 = perimeter of the square
