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

(5x - 7y + 8z) - (-2x + 3y -z)
= 5x - 7y + 8z + 2x - 3y + z ..distribute the negative sign or the negative 1.
= 7x - 10y + 9z .. add like terms.

hope it helped. ^_^

(5x - 7y + 8z) - (-2x + 3y - z)
= 5x - 7y + 8z + 2x - 3y + z
= 5x + 2x - 7y - 3y + 8z + z
= 7x - 10y + 9z

7x -10y + 9z

Just add x, y and z respectively and remember to change the sign of each term inside the second brackets