trig

We can make use of the identity $\cos \theta = \sin(90^\circ - \theta)$.

$\sin 3\theta = \cos 6\theta\\ \sin 3\theta = \sin(90^\circ - 6\theta)$

To get the smallest positive angle theta, we remove the sin from both sides (note that it does not always work, but if we assume that 0 degrees < 3 theta < 90 degrees and 0 degrees < 90 degrees - 6 theta < 90 degrees, then it works.)

$3\theta = 90^\circ -6\theta\\ 9\theta = 90^\circ\\ \theta = \boxed{10^\circ}$