17 + 15 : [(15 - x.3) -4] -11 = 9
17 + 15 : [(15 - x.3) -4] = 9 + 11
17 + 15 : [(15 - x.3) -4] = 20
15 : [(15 - x.3) -4] = 20 - 17
15 : [(15 - x.3) -4] = 3
(15 - x.3) -4 = 15 : 3
(15 - x.3) -4 = 5
15 - x.3 = 5 + 4
15 - x.3 = 9
x.3 = 15 - 9
x.3 = 6
x = 6 : 3
x = 2.