I would start by plugging in a number for x. Just any random number. Let's take 2 for example. Then the equation would go from 2x + y = -4, to 2(2) + y = -4. Then solve.
2(2) + y = -4
4 + y = -4, after you've done that, plug in a number for y, that completes the equation. In this case, the only acceptable number, would be -8. Because it completes the number sentence.
4 + (-8) = -4
-4 = -4.
Now plug in other numbers, to get the same outcome. Hope this helped!
-Twixx