If rx-st=r, which expression represents x

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

Just look outside and then you’ll see where the awnser is