Hello

I've been looking at a question for some time, but haven't been able to find a way to start. The question is:

'If x^2 - px - q = 0, where p and q are positive integers, which of the following could not equal x^3?'

4x + 3, 8x + 5, 8x + 7, 10x + 3, 26x + 5

I know the equation has one positive and one negative root. I've completed the square and found the discriminant, but I can't find a way into the question. Could someone please give me a pointer.

Thank you.