First, we need the coefficient of one of our variables to be the same.
In this case, it's already done for us. We have 6x in both equations.
Now, we just subtract the equations.
[tex]\ \ 6x-3y=30\\\underline{-6x+y=18}\\\overline{} \ \ \ \ -\ 4y=12\\\overline{} \ \ \ \ \ \ \ \ \ \ y=-3[/tex]
Note that the subtraction distributes to each term of that equation.
(you can also do something similar by adding equations--as long as you get rid of of that variable...and if your coefficients aren't the same to begin with, just multiply both sides of one of those equations to get them equal.)
Now, we just use that y = -3 to find x.
[tex]6x+y=18\\6x-3=18\\6x=21\\\boxed{x=3.5}[/tex]
