$\displaystyle f(x) = \int_0^x \frac{ t^2 - 36 }{ 1+\cos^2(t)} dt $

At what value of $\displaystyle x $ does the local max of $\displaystyle f(x) $ occur?

$\displaystyle x = ? $

$\displaystyle f '(x) = \int_0^x \frac{ x^2 - 36 }{ 1+\cos^2(x)} $

im stuck on this part, not sure what to do after.