Hello there!
To start, since you already know that the value of y equals 2x, you can plug 2x in place of y in 3x+3y=-18. This would look like this:
3x+3(2x)=-18
Now simplify:
3x+6x=-18
Now, combine your like terms:
9x=-18
And divide both sides by 9 to solve for x:
x=-2
Now that you have x=-2, you can plug that back into y=2x where you plug in -2 in place of x:
y=2(-2)
y=-4
Therefore, your x=-2 and your y=-4 which would be your final answer.