Remember that the additive inverse are two equal numbers (or expressions) but with different signs.
We have then, that according with the coment below:
(3a2 - 5ab + b2) - (-3a2 + 2b2 + 8ab)
the additive inverse is:
(3a2 - 2b2 - 8ab) Note that is the same expression that
(-3a2 + 2b2 + 8ab)
but with different signs.
Answer:
the additive inverse of the polynomial being subtracted is
(3a2 - 2b2 - 8ab)