How can I find the equation for x
From
(x/y) = (x+a-b)/z
Just do what you always do when solving for a variable. Move things around till it is all alone
x/y = (x+a-b)/z
As usual, just cross-multiply to clear the fractions
xz = y(x+a-b)
xz = xy + y(a-b)
xz-xy = y(a-b)
x(z-y) = y(a-b)
x = y(a-b)/(z-y)