\(-20m^2-280m+160=0\)
\(\Leftrightarrow-20\left(m^2+14m-8\right)=0\)
\(\Leftrightarrow-20\left(m^2+2.m.7+49-57\right)=0\)
\(\Leftrightarrow-20\left(m+7\right)^2+1140=0\)
\(\Leftrightarrow\left(m+7\right)^2=57\)
\(\Leftrightarrow\left[{}\begin{matrix}m+7=\sqrt{57}\\m+7=-\sqrt{57}\end{matrix}\right.\)
\(\Leftrightarrow\left[{}\begin{matrix}m=\sqrt{57}-7\\m=-\sqrt{57}-7\end{matrix}\right.\)
