You need to add one equation to the other. To do this, multiply the divide the entire equation by 2. The resulting equation gives: -6x + 3y = 9
Now, add this to the second. Doing this will cancel out the 6xs because they are opposite signs.
-4y = 4 is your simplified equation.
Solve for y: y = -1
Plug this into an original equation.
-12x + 6(-1) = 18
-12x -6 = 18
-12x = 24
x = -2