Question 187560
I assume you understand the 'multiply' this part. Basically use the same rules you know from arithmetic (distribution, rules of order, etc) to expand all the terms.

Collecting terms means to group like terms and simplify. SO you use assoications and commutation to 'gather up' all the constants (numbers), all the terms with just a single x in them, all the terms with x^2, etc. Once you group them like that, then just add/subtract to simplify

[(x–4)*(2x^2+4)] + [(x+4)*(x^3–1)] – (x^2–x–1)  expand (added [] for effect)
{{{2x^3 +4x - 8x^2 - 16 + x^4-x + 4x^3 -1 - x^2 +x + 1}}} group
{{{x^4 + 2x^3 + 4x^3 - 8x^2 - x^2 + 4x -x +x -16 -1 -1}}} simplify
{{{x^4 + 6x^3 -9x^2 +4x -18}}}