(5x^4 + 3x^3 - 4) - (7x^2 - 9x + 9) =?

Distribute the - sign to the second half of the problem
so
(5x^4 + 3x^3 - 4) - (7x^2 - 9x + 9)
= 5x^4 + 3x^3 - 4 - 7x^2 + 9x - 9
= 5x^4+3x^3-7x^2+9x-13

Yep, its -19. Dont subject approximately having to relearn all that stuff its like driving a bike, youll not at all forget approximately. All that stuff will come decrease back. you probably did the order of operations completely so dont doubt your self.

(5x^4 + 3x^3 - 4) - (7x^2 - 9x + 9)
= 5x^4 + 3x^3 - 4 - 7x^2 + 9x - 9
= 5x^4 + 3x^3 - 7x^2 + 9x - 4 - 9
= 5x^4 + 3x^2 - 7x^2 + 9x - 13

5x^4 + 3x³ - 7x² + 9x - 13 = 0

Your question is incorrect, as the '?' could equal anything, as the x could be any number.
Could you be trying to simplify? If so, the answer would be:
5x^4 + 3x^3 - 4 - 7x^2 - 9x + 9 = ?
5x^4 + 3x^3 + 7x^2 + 9x - 13 = ?
(The brackets mean little)

Hmm, solving for x is a very messy opporation. It's taken me a while, and I'm sure it's not what your after, but if you solve for x, you'll get two possible solutions.

They are: x= -2.033 and x= 1.108.

I would advise you rephrase your question, or perhaps you read the question wrong in the first place?

(5x^4 + 3x^3 - 4) - (7x^2 - 9x + 9)

= 5x^4 + 3x^3 - 7x^2 + 9x - 4 - 9

= 5x^4 + 3x^3 - 7x^2 + 9x - 13 (ans)