Given that the quadratic function y = ax & # 178; + BX + C has the maximum value and is negative, then a =?. C =?

Given that the quadratic function y = ax & # 178; + BX + C has the maximum value and is negative, then a =?. C =?

If the function has a maximum value, then a is negative, and the maximum value is 4ac-b ^ 2 / 4A = C - (b ^ 2 / 4A) is negative, then C is also negative
So a and C are all negative numbers

Given that the image of quadratic function y = AX2 + BX-1 passes through {2, - 1}, and the minimum value of this function is - 3, find the relation of this function
I made a system of equations - 4a-b2 / 4A 4A + 2b-1 = - 1. I can't work it out, but this equation is wrong

Substituting point {2, - 1} into y = AX2 + BX-1, we get a = - B / 2 (4ac-b ^ 2) / 4A = - 3, that is - 4a-b ^ 2 = - 12a, 4A ^ 2-8a = 0, we get a = 2 and a = 0 (rounding off), then B = - 4, so the relation of function y = 2x ^ 2-4x-1