what equals 0 if the equation to the parabola is y=-16x2+28x+5

y=-16x²+28x+5   = 0

This won't factor.....using the on-site solver [and the quadratic formula] , we have

$${\mathtt{\,-\,}}{\mathtt{16}}{\mathtt{\,\times\,}}{{\mathtt{x}}}^{{\mathtt{2}}}{\mathtt{\,\small\textbf+\,}}{\mathtt{28}}{\mathtt{\,\times\,}}{\mathtt{x}}{\mathtt{\,\small\textbf+\,}}{\mathtt{5}} = {\mathtt{0}} \Rightarrow \left\{ \begin{array}{l}{\mathtt{x}} = {\mathtt{\,-\,}}{\frac{\left({\sqrt{{\mathtt{69}}}}{\mathtt{\,-\,}}{\mathtt{7}}\right)}{{\mathtt{8}}}}\\
{\mathtt{x}} = {\frac{\left({\sqrt{{\mathtt{69}}}}{\mathtt{\,\small\textbf+\,}}{\mathtt{7}}\right)}{{\mathtt{8}}}}\\
\end{array} \right\} \Rightarrow \left\{ \begin{array}{l}{\mathtt{x}} = -{\mathtt{0.163\: \!327\: \!982\: \!864\: \!759\: \!4}}\\
{\mathtt{x}} = {\mathtt{1.913\: \!327\: \!982\: \!864\: \!759\: \!4}}\\
\end{array} \right\}$$