f(x) = x
2 + bx + c
f ( 2,5 ) = 2.5^2 + 2.5 * b + c = 0
f ( 0 ) = 0^2 + 0 * b + c = -5
f ( 0 ) = c = -5
f ( 2,5 ) = 2.5^2 + 2.5 * b + c = 0
f ( 2,5 ) = 2.5^2 + 2.5 * b -5 = 0
6.25 + 2.5 * b -5 = 0
2.5 * b = 5 - 6.25
b = -0.5
f ( x ) = x^2 + -0.5 * x - 5