The missing term represented by the question mark

Hi there!

You might want to simplify them first.
I'll make the ? into x.

(-b³ + 3b² + 8) - (x - 5b² - 9) = 5b³ + 8b² + 17
-b³ + 3b² + 8 - x + 5b² + 9 = 5b³ + 8b² + 17

add the like terms
-b³ + 8b² + 17 -x = 5b³ + 8b² + 17

You can see that the 8b² and 17 are equal.
To get 5b³, the x has to be 6b³.
-b³ + 6b³ = 5b³.

6b³ has to be negative because we have a minus sign in front of the parenthesis.

So your missing term would be -6b³.
Hope this helped! :)