To figure this out, you would first isolate one of the variables in one of the two equations given.
I will first isloate the second equation's Y by subratcting it on both sides to get it onto the other side, thus not changing any of the values in the equation, just moving it around.
The equation would be: x=0+y or simply x=y
You plug in the x to the first equation with parenthesis around it to get it all into on equation: (y)+3y=-4
Add the two like terms, y and 3y to get 4y=-4
Divide that equation by four on both sides to get y=-1
Plug -1 into the second equation that you had moved around (x=y) to get x=-1
So -1=y and -1=x
