Chapter 12 - Sequences; Induction; the Binomial Theorem - 12.5 The Binomial Theorem - 12.5 Assess Your Understanding - Page 836: 37

$314928x^7.$

According to the Binomial Theorem, the nth term in the expansion of $(bx+a)^k$ is given by: ${k\choose k-n+1}a^{n-1}(bx)^{k-n+1}.$ Hence here for $n=3,k=9$: ${9\choose 9-3+1}(-2)^{3-1}(3x)^{9-3+1}={9\choose 7}(-2)^{2}(3x)^{7}=314928x^7.$