（x-2y)(x+4y) =x^2+2xy-8y^2
(ab+7)(ab-3) =a^2b^2+4ab-21
n(n+2)(2n+1) =(n^2+2n)(2n+1)=2n^3+5n^2+2n
(1-3x)(1+2x)-3x(2x-1)=1-x-6x^2-3x^2+3x=1+2x-9x^2
3ab[6ab-3(6ab-3(ab-1/3a^2b)]
= 3ab[6ab-3(6ab-3ab+ a^2b)]
=3ab[6ab-3(3ab+a^2b)]
=3ab[6ab-9ab-3a^2b]
=3ab[-3ab-3a^2b]
=-9a^2b^2-9a^3b^2
3xy(x-2xy-xy^2z)=3x^2y-6x^2y^2-3x^2y^3z
(5ab-2)(-3a^2b) =-15a^3b^2+6a^2b
(5ab-2)(-3a^2b) =-15a^3b^2+6a^2