solve the equation f'(x) = 0:

Break it down gently
f(x) = 1 - sin⁴3x + cos6x/6
let's break ( - sin⁴3x) and (Cos6x/6) and 1 separately
the derivative of ( -sin⁴3x) = 12cos3xsin³3x
the derivative of (cos6x/6) = -sin6x
the derivative of (1) = 0
f'(x) = 0
12cos3xsin³3x - sin6x = 0