1) Let's multiply the second equation by -2, to eliminate x
2x -3y=-11
x+3y = 8
2) Now let's add both equations
2x -3y=-11
-2x-6y = -16
--------------------
-9y = -27
3) Divide both by -9
y=3
4) Plug y=3 into the simpler original equation, to make calculations easier:
x +3y=8
x+3(3) =8
x +9 =8 Subtract 9 from both sides
x = 8-9
x=-1
So, after multiplying it we add both equations to eliminate one variable either x or y.