no 17 tolong dijawab.. bagaimana cara penyelesaiannya ?

S1 = U1
S2 = U1 + U2

S1 = 1² - 4.1 = -3
S2 = 2² - 4.2 = -4

U2 = S2 - U1 = -4-(-3) = -1

a = -3
U2 = a + b
 -1 = -3 + b
b = 2

U7 = a + 6b
      = -3 + 6(2)
      = -3 + 12
      = 9 (C)