Let $u = 3x$. Then, $\frac{du}{dx} = 3$, so $dx = \frac{1}{3} du$. Now, we can rewrite the integral in terms of $u$:
$$
\int \sin 3x dx = \int \sin u \frac{1}{3} du
$$
Now, we can integrate with respect to $u$:
$$
\int \sin u \frac{1}{3} du = \frac{1}{3} \int
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