Chapter 12 - Sequences; Induction; the Binomial Theorem - 12.5 The Binomial Theorem - 12.5 Assess Your Understanding - Page 836: 38

$48384x^3.$

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 $n=6,k=8$: ${8\choose 8-6+1}(2)^{6-1}(3x)^{8-6+1}={8\choose 3}(2)^{5}(3x)^{3}=48384x^3.$