Find h(x), with terms in order of decreasing degree, if
Let's take this term by term. We have 3x^4+2x-1 and 5x^2-6x-1. h(x) must have -3x^4 because all x^4 terms cancelled in the sum, 5x^2 because there was no x^2 term before so this must have been added, and -8x because 2x-8x=-6x.. So h(x) is -3x^4+5x^2-8x.