The subtraction sign in front of the second set of parenthesis changes the signs on the terms inside those parenthesis. So this could be rewritten as [tex]x-3x^4+2x^2+x^3-4x^6[/tex]. None of the exponents on the x are the same, so we will just express it in terms of descending powers of x. [tex]-4x^6-3x^4+x^3+2x^2+x[/tex]. This is also the same as [tex]4x^6+3x^4-x^3-2x^2-x[/tex] if you don't like to start with a negative. Either one is correct.
