x + y = 8
solve for y, y = -x + 8
put this into the 2nd equation for y
x - 3(-x + 8) = -4
distribute
x + 3x - 24 = -4
combine like terms
4x - 24 = -4
add 24 to both sides
4x = 20
divide both sides by 4 and x = 5
the put 5 in for x and solve for y. y = 3
answer is (5,3)