Replace x in the second equation with the value of x from the first one.
x = -3y
x + y = -6
-3y + y = -6
Simplify:
-2y = -6
Divide both sides by -2:
y = -6 / -2
y = 3
Now replace Y with 3 in one of the equations to solve for x:
x = -3y = -3(3) = -9
X = -9, Y = 3
(-9,3)