参考・概略です
公式の利用
(a+b)(a²-ab+b²)=a³+b³
(a-b)(a²+ab+b²)=a³-b³
(1) (x+2)(x²-2x+4)
=(x+2){(x)²-(x)(2)+(2)²}
=(x)³+(2)³
=x³+8
(2) (x-3)(x²+3x+9)
=(x-3){(x)²+(x)(3)+(3)²}
=(x)³-(3)³
=x³-27
(3) (x+3y)(x²-3xy+9y²)
=(x+3y)((x)²-(x)(3y)+(3y)²)
=(x)³+(3y)³
=x³+27y³
(4) (2x-3a)(4x²+6ax+9a²)
=(2x-3a)((2x)²+(2x)(3a)+(3a)²)
=(2x)³-(3a)³
=8x³-27a³