Maximum
It is a maximum point when the coefficient of x^2 is negative or when the graph curves downward, we have the maximum value.
[tex]\begin{gathered} \text{Maximum value = }\frac{4ac-b^2}{4a} \\ a\text{ = -1} \\ b\text{ = 8} \\ c\text{ = -20} \\ \text{Maximum value = }\frac{4\times(-1)\times(-20)-8^2}{4\times(-1)} \\ =\text{ }\frac{80\text{ -64}}{-4} \\ =\text{ }\frac{16}{-4} \\ =\text{ -4} \end{gathered}[/tex]Second Method
You can also find the maximum point from the graph at the turning point.
i.e the graph turn at (4,-4)
Hence, the maximum value is the y-coordinate at y = -4
