Start off with X. If X=5+Y and X+Y=3Y-3 set up a system of equations problem to solve. You can plug in 5+Y for X in the second equation. 5+Y+Y=3Y-3. Combine like terms and 5+2Y=3Y-3.
Isolate the variable and you get Y=8. The smaller number is 8 and the larger number is 13.