Find the smallest positive value which satisfies x = 2.600cos(2.400x)

I used the formula

xn+1 = xn + [2.600cos(2.400xn)-xn]/[6.240sin(2.400xn)-1]

I ended up with x = 0.4900 after a few runs through but my online homework tells me this is incorrect. My calculator seems to get close to this when I use the zoom function to check though.