(2u3 - 7u + 4u2 - 9) - (-7u + 2u3 - 9 + 4u2)

Answer 1

(2u^3 - 7u + 4u^2 - 9) - (-7u + 2u^3 - 9 + 4u^2)
If you rearrange these expressions so that the powers are in descending order, I think you will find it quite easy.