If \[\frac{x-y}{x+y} = \frac52,\] find $\frac xy.$
2x - 2y = 5x + 5y
-3x = 7y
x = -(7/3)y
so x/y = -7/3.
Check: we can actually solve this equation : x - y = 5, and x + y = 2. Add the 2 equations, and get 2x = 7, so x = 3.5, so y = -1.5. x / y = 3.5 / -1.5 = -7/3.
