The first step in simplifying this expression is to identify which, if any, terms can be combined. Remember, only like terms — constant terms or terms with the same variable and the same exponent — can be combined.
( 3z^3 - 4z + 7z^2)+( 8z^2 - 6z - 5)
In this case we have one z^3-term, two z^2-terms, two z-terms, and one constant.
Only the z^2-terms and z-terms can be combined, so to simplify the expression we will remove parentheses, rearrange it according to the Commutative Property of Addition, and then combine like terms.