Answer:
11a^3 + 7a^2 + 8a
Step-by-step explanation:
( 5a^3 - 2a^2 ) + ( 6a^3 + 9a^2 + 8a )
= 5a^3 + 6a^3 + -2a^2 + 9a^2 + 8a
= 11a^3 + 7a^2 + 8a