x + y =0 (solve this for y, you can also solve 3x + y=4 for y but in my opnion the first one is easier ) subtract x from both sides
y = -x (substitute this one for y in the other equation)
3x - x = 4 (3x - 1x= 2x)
2x = 4 ( divide each side by 2)
x = 2 ( this is one answer, plug this in to find the y value)
2 + y = 0 ( subtract 2 from each side)
y = -2
x = 2 y = -2 is your answer
check: 3(2) +(-2) = 4 6 + -2 = 4 this checks out
-2 + 2 = 0 this also checks out the answer is correct.