a) \(\left(x^3-2y\right)^3\)
\(=\left(x^3-2y\right)\left[\left(x^3\right)^2+x^3.2y+\left(2y^2\right)\right]\)
\(=\left(x^3-2y\right)\left(x^6+2x^3y+4y^2\right)\)
b) \(\left(x-3y\right)\left(x^2+3xy+9y^2\right)\)
\(=\left(x-3y\right)\left(x^2+x.3y+\left(3y\right)^2\right)\)
\(=\left(x-3y\right)^3\)