to cancel out the X's find the least common multiple of 5 and 4:
multiples of 5: 5, 10, 15, 20, 25, 30
multiples of 4: 4, 8, 12, 16, 20, 24
the least common multiple of 5 and 4 is 20,
so we need to multiply 5x and 4 x by numbers that would make-20 and 20 ( -20+20 = 0) which would cancel them out
5 x -4 = -20
4 x 5 = 20
-20 + 20 = 0
the 3rd answer would cancel out the x's
