You can start by multiplying the first equation by twelve, by doing this you get whole numbers. This yields:
4x + 3y =12
2x - 3y = -30
As you can see, you have two terms that only differ in sign.
By adding both equations together you get this:
6x=-18 or x=-3
The reason why you can add these equations together is illustrated in the next example:
4=x
5=y
4+5=x+y
Simple no?
Now that you now x you can substitute the value in one of the equations and you get the y value:
4*-3 + 3y =12
-12 + 3y =12
3y= 24
y=8
