\(\begin{aligned}&\frac{ax}{x+y}-a\\=\,&a\frac{x}{x+y}-a\\=\,&a\left(\frac{x}{x+y}-1\right)\\=\,&a\left(\frac{x}{x+y}-\frac{x+y}{x+y}\right)\\=\,&a\frac{x-\left(x+y\right)}{x+y}\\=\,&a\frac{x-x-y}{x+y}\\=\,&a\frac{-y}{x+y}\\=\,&-a\frac{y}{x+y}\\=\,&-\frac{ay}{x+y}\end{aligned}\)