(3x+1)(3x^2 +7x−7)

= 3x(3x^2 + 7x - 7) + 1(3x^2 + 7x - 7)

= 9x^3 + 21x^2 - 21x + 3x^2 + 7x - 7
= 9x^3 + 24x^2 - 14x - 7