SAT Question 668f1863: Nonlinear Functions answer and explanation with Desmos solution

It's given that function $f$ is a quadratic function where $f(-20)=0$ and $f(-4)=0$. It follows that the graph of $y=f(x)$ in the xy-plane passes through the points $(-20,0)$ and $(-4,0)$. When the graph of a quadratic function contains two points $(a,0)$ and $(b,0)$, the x-coordinate of the vertex of the graph is the average of $a$ and $b$. Therefore, the x-coordinate of the vertex of the graph of $y=f(x)$ is $\frac{-20+\left(-4\right)}{2}$, or $-12$. It's given that the graph of $y=f(x)$ in the xy-plane has a vertex at $(r,-64)$. It follows that the value of $r$ is $-12$.