(x + y) / (y - x) - (y - 3*x) / (y - x)

[latex] \frac{x+y}{y-x}- \frac{y-3x}{y-x}= \frac{x+y-(y-3x)}{y-x}= \frac{x+y-y+3x}{y-x}= \frac{4x}{y-x} [/latex] [latex] \frac{x+y}{x-y}- \frac{y-3x}{y-x}= \frac{x+y}{x-y}+ \frac{y-3x}{x-y}= \frac{x+y+(y-3x)}{x-y}= \frac{x+y+y-3x}{x-y}= \frac{2y-2x}{x-y}= \frac{2(y-x)}{x-y}=-2[/latex]