We pick to eliminate y.
So we will multiply the second equation by 3 and then we add the two equations.

3x + 6y = -15
6x - 2y = 12

After we multiply the second equation by 3 we get:

3x + 6y = -15
18x - 6y = 36

Add the two equations:

3x+18x=-15+36

21x=21

So

x = 1

We replace x with 1 in 6x - 2y = 12

And we get:
6 - 2y = 12

-2y=12-6
-2y=6
y=-3

So:
x=1 and y=-3