Expand and simplify (x+3) (x-3) (3x+2)

(x+3) (x-3) results in a product which is the difference of two squares: x^2 - 3^2.

Multiply this (x^2 - 3^2) by the third factor, (3x+2):

3x^3 - 9x + 2x^2 - 3^2*2, or, in descending order by powers of x,

3x^3 + 2x^2 - 9x - 18

I think it would be 3x³ + 2x² - 27 - 18