If $700 is the amount after 20% has been deducted, then $700 represents 80% of the original rent. If we call the original amount R, then 700=80% of R or 80R/100=0.8R. If 0.8R=700, then R=700/0.8=$875.
To check your answer, work out 20% of $875=875/5=175 because 20%=20/100=1/5. If we take $175 off $875 we get $700, which is the reduced rent.
Instead of writing x-20%=$700, you should have written x-20% of x = $700 or x-0.20x=$700; this is the same as x(1-0.2)=$700, which is 0.8x=$700, and the answer would have been easy! So you were nearly there.