6. $-5xy^2(2x^2y+3xy^2)=-10x^3y^3-15x^2y^4$
7. $2xy(3x^2y-0,5xy^3)=6x^3y^2-x^2y^4$
8. $-3xy^2(4x^2-xy+2y^2)=-12x^3y^2+3x^2y^3-6xy^4$
9. $3x^{n-2}y^{n-1}(x^{n+2}-2x^{n+1}y^n+y^{n+1})=3x^{2n}y^{n-1}-6(xy)^{2n-1}+3x^{n-2}y^{2n}$
10. $4x^2(7x^2+3)-6x(3x^3-2x+1)-5x^3(2x-1)$
$=28x^4+12x^2-18x^4+12x^2-6x-10x^4+5x^3$
$=5x^3+24x^2-6x$
11. $\dfrac{3}{2}x\Bigg(x^2-\dfrac{1}{3}x+2\Bigg)-\dfrac{5}{2}x^2\Bigg(x+\dfrac{1}{5}\Bigg)$
$=\dfrac{3}{2}x^3-\dfrac{1}{2}x^2+3x-\dfrac{5}{2}x^3-\dfrac{1}{2}x^2$
$=-x^3-x^2+3x$
