To get x/y, you need to know the values of "x" and "y".
So, we'll need 2 equations.
Where are they?
Let's see, you have a ratio isn't it?
You can make two equn: out of them.
e.g. in 1/3, multiplying both numerator and denominator with 4 will give you 4/12. So, you can write as 1 x 4 = 4 and 3 x 4 = 12.
Similarly, in the given ratio
(x+y)/(x-y) = a/b, we may write
x + y = ak -----(1)
x - y = bk -----(2) where "k" is a constant.
equn: (1) + (2) ---> 2x = k (a+b)
therefore, x = k(a+b)/2
and you will get y = ak - k(a+b)/2
y = k (a-b)/2
Therefore, x/y = (a+b)/(a-b)