Question 568308
The remaining area will just be the current area minus how much is cut out.

Its hard to tell from your working if x is the side or x is the area. If x is the area you just get
r=120-x where r is the remaining area, 120 is the current area and x is the area of the square

if its the side than you have
{{{r=120-x^2}}} since the area of a square is just the side squared.