a]
[tex]x(x^2-1)+2x(4+x^2)-3(4x-x^2)=x^3-x+8x+2x^3-12x+3x^2=\\\\4x-19x[/tex]
b] [tex]x^2(y-xy)-y(x^2-xy^2)=x^2y-x^3y-x^2y+xy^3=-x^3y+xy^3[/tex]
c] [tex]-\frac{3}{4}b(-4a^2-8ab)=3a^2b+6ab^2[/tex]
