[tex]\bf (-y+4)(2y-1)\implies \stackrel{(-y)(2y-1)}{-2y^2+y}~~+~~\stackrel{(4)(2y-1)}{8y-4}\implies -2y^2+9y-4[/tex]
notice, just like in any multiplication, you can simply multiply each term of either polynomial times the other, sum them up and then cancel like-terms.