$(\color{red}{-7y}+\color{green}{8})^2
\,$ $=\,(\color{red}{-7y})^2 \,+ 2\times\color{red}{(-7y\,)}\times\color{green}{8} \,+ \color{green}{8}^2$ $= $ ?
$-7\,y^2\, -112\,y\,+ 64$ $-49\,y^2\, -112\,y\,+ 64$ $49\,y^2\, -112\,y\,+ 64$ $49\,y^2\, +112\,y\,+ 64$