$(\color{red}{9y}+\color{green}{x})^2
\,$ $=\,(\color{red}{9y})^2 \,+ 2\times\color{red}{9y}\times\color{green}{x} \,+ \color{green}{x}^2$ $= $ ?
$81y\, +18\,x\,y\,+ \,x^2$ $81\,y^2\, +18\,x\,y\,+ \,x^2$ $9\,y^2\, +18\,x\,y\,+ \,x^2$ $-81\,y^2\, +18\,x\,y\,+ \,x^2$