Find any critical numbers for f(x) = 2sec(x) + tan(x), 10 pi < x < 12 pi.

When I derived it I got: f'(x) = 2(sec(x))(tan(x)) + sec^{2}(x)

I simplified it to: f'(x) = sec(x)(2tan(x) + sec(x))

I need help with what to do after this step.

Thank you for your help in advance!