[tex]\bf \qquad \qquad \textit{vertex of a parabola}\\ \quad \\
\begin{array}{llllll}
f(x)&=&-1x^2&+8x&-21\\
&&\uparrow &\uparrow &\uparrow \\
&&a&b&c
\end{array}\qquad
\left(-\cfrac{{{ b}}}{2{{ a}}}\quad ,\quad {{ c}}-\cfrac{{{ b}}^2}{4{{ a}}}\right)[/tex]
the squared variable is the "x", thus is going vertical, down or up? well
the leading term's coefficient is negative, that means, down
