No.12 ya pake caranya

p = (3x + 4) cm
l = (2x + 4) cm

Keliling = 36 cm
2p + 2l = 36
2 (3x + 4) + 2 (2x + 4) = 36
6x + 8 + 4x + 8 = 36
10x + 16 = 36
10x = 36 - 16
10x = 20
x = 20/10
x = 2