7a + (8b) + (-15a) + 9b + (-4c) + 8=

-8a+17b-4c+8

steps:

when there is a "+" in front of the parenthesis the expression remains the same.

7a+8b-15a+9b-4c+8

Collect the like terms

7a+8b-15a+9b-4c+8

7a and 15a

8b and 9b

15a-7a = - 8a

9b+8b = 17b

Final answer:

-8a+17b-4c+8

7a + (8b) + (-15a) + 9b + (-4c) + 8

7a+8b-15a+9b-4c+8 (changing some of the symbols)

7a-15a+8b+9b-4c+8

=-8a+17b-4c+8

Answer: - 8a+17b-4c+8