Add.

(5c^4-3c^3+4c^2+15c-6)+(c^5+9c^3+6c^2-4c+2)+(-8c^4+c^2-7c-8)

To add the given algebraic expressions, you need to combine like terms. Like terms are terms that have the same variable(s) raised to the same exponent(s). Let's rearrange and group the terms together based on their exponents:

(5c^4 - 8c^4) + (-3c^3 + 9c^3) + (4c^2 + 6c^2 + c^2) + (15c - 4c - 7c) + (-6 + 2 - 8)

Now, let's combine the like terms:

(-3c^3 + 9c^3) simplifies to 6c^3
(4c^2 + 6c^2 + c^2) simplifies to 11c^2
(15c - 4c - 7c) simplifies to 4c
(-6 + 2 - 8) simplifies to -12

Putting it all together, the simplified expression is:

6c^3 - 3c^4 + 11c^2 + 4c - 12