[tex]~~~~~~\textit{vertical parabola vertex form} \\\\ y=a(x- h)^2+ k\qquad \begin{cases} \stackrel{vertex}{(h,k)}\\\\ \stackrel{"a"~is~negative}{op ens~\cap}\qquad \stackrel{"a"~is~positive}{op ens~\cup} \end{cases} \\\\[-0.35em] ~\dotfill\\\\ \stackrel{vertex}{(2~~,~~-4)}\qquad \implies y=a(x-2)^2-4\qquad \textit{we also know that} \begin{cases} y=-3\\ x=-3 \end{cases} \\\\\\ -3=a(-3-2)^2-4\implies 1=a(-5)^2\implies 1=25a\implies \boxed{\cfrac{1}{25}=a}[/tex]
