Answer & Step-by-step explanation:
To simplify the expression 3a + 6b - 8a + 9b, we can combine the like terms (terms with the same variable).
First, we can combine the 3a and -8a terms since they have the same variable 'a'. When we subtract 8a from 3a, we get -5a.
Then, we can combine the 6b and 9b terms since they have the same variable 'b'. When we add 6b and 9b, we get 15b.
Putting it all together, we get:
3a + 6b - 8a + 9b = -5a + 15b
Therefore, the simplified expression is -5a + 15b.