Answer :
Width of a rectangle is half its length:
2w = l
Perimeter is 36
2w + 2l = 36
Plug first equation into second (substitution)
2w + 2(2w) = 36
2w + 4w = 36
6w = 36
w = 6
Plug back into first equation to solve for l
2(6) = 12 = l
2w = l
Perimeter is 36
2w + 2l = 36
Plug first equation into second (substitution)
2w + 2(2w) = 36
2w + 4w = 36
6w = 36
w = 6
Plug back into first equation to solve for l
2(6) = 12 = l