a)
(3x+1)(9x^2+1)(1-3x)=(1-3x)(3x+1)(9x^2+1)=
=-(3x-1)(3x+1)(9x^2-1)=-[(3x)^2-1^2] (9x^2+1)=
-(9x^2-1)(9x^2+1)=-[(9x^2)^2-1^2)]=-(81x^4-1)=1-81x^4
b)
(x-3)(x+3)(2x^2+18)=(x^2-9)*2(x^2+9)=2(x^2-9)(x^2+9)=
=2[(x^2)^2-9^2]=2(x^4-81)=2x^4-162
-----------
(a-b)(a+b)=a^2-b^2