Question

If rx-st=r, which expression represents x

Answer 1

To get an expression relating to x you need to have x alone on one side of your equation. ie x = 
Try this: 
rx - st = r 
remove st from the rx side 
rx = r + st 
now divide by r 
x = (r + st)/r 
You now have your expression relating to x

Answer 2

The correct answer is:

x = (r+st)/r; or

x = 1+ (st)/r.

Explanation:

To isolate the variable x, we must cancel everything else on that side of the equals sign.

We begin with st, since it is farther from x. Add st to each side:

rx - st = r

rx - st + st = r + st

rx = r + st

Now we divide each side by r:

rx/r = (r+st)/r

x = (r+st)/r = r/r + st/r = 1 + (st)/r