Finding x when f'(x) = 0,

From f(x)=(25e^-0.5x)(cos(x-0.464) + 6

Using product rule I have attained the derivative

f'(x)= (-12.5e^-0.5x)(cos(x-0.464)) + (25e^-0.5x)(-sin(x-0.464))

When f'(x) = 0, how do I find x?