Determine the extreme values of the following function in the given interval.

f(x)=2sin4x+3 , XE [0, pi]

This is my work:

f'(x)=2cos4x (4)

f'(x)=8cos4x

Now I am trying to find the critical numbers, but I'm not sure how to solve for x.

f'(x)=0

cos4x=0

x=22.5

Is this right?