(2a-3)(4a^2+6a+9)

$$\\(\textcolor[rgb]{0,1,0}{2a}\textcolor[rgb]{0,0,1}{-3})(4a^2+6a+9)\\\\
=\textcolor[rgb]{0,1,0}{2a}(4a^2+6a+9)\textcolor[rgb]{0,0,1}{-3}(4a^2+6a+9)\\\\
=[\textcolor[rgb]{0,1,0}{2a}(4a^2+6a+9)]+[\textcolor[rgb]{0,0,1}{-3}(4a^2+6a+9)]\\\\
=8a^3+12a^2+18a\;\;\;+\;\;\;-12a^2-18a-27\\\\
=8a^3+12a^2+18a\;\;\;\;\;\;\;-12a^2-18a-27\\\\$$

and you can finish it.