Add :

8x - 3y + 7z, - 4x + 5y - 4z, - x - y - 2z

8x - 3y + 7z + (- 4x + 5y - 4z )+ (- x - y - 2z)

= 8x - 3y + 7z - 4x + 5y - 4z - x - y - 2z

Arrange the polynomial with like terms together and combine them.

= (8x - 4x - x) + (- 3y + 5y - y) + (7z - 4z - 2z)

= 3x + y + z

Hence, the addition of 8x - 3y + 7z, - 4x + 5y - 4z, - x - y - 2z is 3x + y + z.