check the picture below.
[tex]\bf \textit{ vertex of a vertical parabola, using coefficients}\\\\
\begin{array}{lccclll}
P(x) = &{{ -0.05}}x^2&{{ +500}}x&{{ -100000}}\\
&\uparrow &\uparrow &\uparrow \\
&a&b&c
\end{array}\qquad
\left(-\cfrac{{{ b}}}{2{{ a}}}\quad ,\quad {{ c}}-\cfrac{{{ b}}^2}{4{{ a}}}\right)\\\\
-------------------------------\\\\
\textit{so }-\cfrac{b}{2a}\textit{ units maximize the profit, and}\\\\\\
c-\cfrac{b^2}{4a}\textit{ dollars is the maximum profit}[/tex]
