Consider the equation rx-sx+y=z; solve for x

So you just rewrite the equation to be equal to x so you need x on a side by itself.

rx-sx+y=z

First, subtract y.

rx-sx=z-y

Second, factor out the x.

x(r-s)=z-y

Lastly, divide (r-s) on both sides.

x= (z-y)/(r-s)