Find the arc length:

$\displaystyle y = 3 + \frac{1}{3} cosh(3x)$ for $\displaystyle 0 \leq x \leq 3$

I got for my integral:

$\displaystyle \int\limits^{3}_{0} \sqrt{1 + sinh^2(3x)} dx$

$\displaystyle = \int\limits^{3}_{0}1 + sinh(3x) dx$

What do I do from here? Should I substitute w/ double angle?