we have
3x + y = 0-------> equation A
x - y = 4
x=y+4 ------> equation B
substitute equation B in equation A
3(y+4)+y=0
solve for y
3y+12+y=0
4y=-12
y=-3
Find out the value of x
x=y+4
substitute the value of y
x=-3+4
x=1
therefore
the solution is (1,-3)