Question

if rx-st=r which expression represents x

Answer 1

Okay, to get an expression that represents X you need it to have X alone, sort of like something that defines X

Answer 2

so, if we remove st from the rx side and make rx = r + st

Answer 3

and then we divide by R to get x = (r + st)/r

Answer 4

x = (r + st)/r represents X, because it states that X = (Things)

Answer 5

So, your final answer is: x = (r + st)/r