After substituting:
[tex]\frac{\sqrt{36sec^2 \theta - 36}}{6 sec \theta}[/tex]
Factor out the 36, allowing the 6's to cancel
[tex]\frac{ \sqrt{sec^2 \theta - 1}}{sec \theta} [/tex]
Use trig identity:
sec^2 - 1 = tan^2
[tex]\frac{\sqrt{tan^2 \theta}}{sec \theta} = \frac{tan \theta}{sec \theta} = sin \theta[/tex]
