h = 3dm
a = 12 cm = 1,2 dm
b = 36 cm = 3,6 dm
P = (a+b)/2 * h = (1,2dm + 3,6dm)/2 * 3dm =
4,8dm/2 * 3dm = 2,4 dm * 3dm = 7,2 dm (do kwadratu)
