1) Why did you give up? Don't do that.
2) Why did you change from "r" to "R"? Don't do that. It makes a difference.
3) Fix your notation. -3-r/2-5 is very different from (-3-r)/(2-5). Write clearly.
4) Why did you create that equation? Understanding what you are doing is very helpful. Solve for 'r'. Isn't that what the problem statement demanded?