Answer:
Yes. This is a function.
Explanation:
Given is a function as

We find that for every x in the domain of real numbers there is a unique image for every x as sin 3x.
Thus this relation given satisfies the requirements of a function.
Also we find that this function is periodic with amplitude 1 and period = 2pi/3
So vertical line test if done will find that there is only one intersection for every vertical line throughout the domain
Hence this relation is definitely a function