How to calculate the value range of mathematical quadratic function

How to calculate the value range of mathematical quadratic function

The general formula of quadratic function is y = ax ^ 2 + BX + C, which is formulated as vertex formula
Y = a (x + B / 2a) ^ 2 + (4ac-b ^ 2) / (4a)
a> 0, opening upward Y > = + (4ac-b ^ 2) / (4a)
a