$(\color{red}{5y}+\color{green}{x})^2
\,$ $=\,(\color{red}{5y})^2 \,+ 2\times\color{red}{5y}\times\color{green}{x} \,+ \color{green}{x}^2$ $= $ ?
$25\,y^2\, +10\,x\,y\,+ \,x^2$ $25y\, +10\,x\,y\,+ \,x^2$ $-25\,y^2\, +10\,x\,y\,+ \,x^2$ $5\,y^2\, +10\,x\,y\,+ \,x^2$