Answer
$2835x^3.$
Work Step by Step
According to the Binomial Theorem, the nth term in the expansion of $(x+a)^k$ is given by: ${k\choose k-n+1}a^{n-1}x^{k-n+1}.$
Hence here for $n=5,k=7$: ${7\choose 7-5+1}3^{5-1}x^{7-5+1}={7\choose 3}3^{4}x^{3}=2835x^3.$
