If it helps do a u-sub where u=6theta
Or you could think of it like this:
If we take the derivative of [tex]\frac{d}{d \theta}(-cos(\theta)) = sin(\theta)[/tex]. That's sort of like the anti-derivative which you seek.
If we take the derivative of [tex]\frac{d}{d \theta}(-cos(6 \theta)) = 6sin(6 \theta)[/tex].
So from there it's easy to see that[tex]\int\sin(6\theta) d\theta = \frac{-cos(6\theta)}{6}[/tex]