c(x) = Ax^2 +Bx +C
1) 15.5 = A (3^2) +B(3) + C
15.5 = 9A + 3B + C
2) 23.5 = A(7^2) + B(7) + C
23.5 = 49A + 7B
3) 56 = A(12)^2 + B(12) + C
56 = 144C +12B + C
2) - 1) ==> 8 = 40A +4B ==> 2 = 10A + B ==> B = 2 - 10A
3) - 2) ==> 32.5 = 95A + 5B ==> 6.5 = 19 A + B ==> B = 6.5 - 19A
2 - 10A = 6.5 - 19A
19A -10A = 6.5 - 2
9A = 4.5
A = 4.5/9
A = 0.5
B = 2 - 10A = 2 - 10(0.5) = 2 - 5 = -3
C = 15.5 - 9A - 3B
C = 15.5 - 9(0.5) - 3(-3) = 20
c(x) = Ax^2 + Bx + C
c(x) = 0.5x^2 -3x + 20