Solution
[tex]\begin{gathered} \text{ since }y=ax^2 \\ \\ \text{ at }(-2,-8) \\ \\ \Rightarrow-8=a(-2)^2 \\ \\ \Rightarrow-8=a(4) \\ \\ \Rightarrow a=-\frac{8}{4}=-2 \\ \\ \Rightarrow y=-2x^2 \end{gathered}[/tex]The quadratic equation is
[tex]y=-2x^2[/tex]