Recall that when adding polynomials you need to combine "like terms" (that is the terms that contain the SAME VAriable (letter) part.
Then in the addition:
(x + 2 y - 4 z) + ( x - 4 y + 6 z)
we combine:
x + x = 2 x
2 y - 4 y = - 2 y
and
- 4 z +6 z = 2 z
obtaining finally:
2 x - 2 y + 2 z
