(−5b 2 −8b)−(−9b 3 −5b 2 −8b) combine like terms

First distribute the -1 coefficient through the second set of parentheses

-5b^2-8b+9b^3+5b^2+8b  now combine like terms

9b^3+(5b^2-5b^2)+(8b-8b)  which gives you:

9b^3