Answer
$1.1\times 10^{45}~photons/s$
Work Step by Step
We can find the energy of each photon:
$E = \frac{hc}{\lambda}$
$E = \frac{(6.63\times 10^{-34}~J\cdot s)(3.0\times 10^8~m/s)}{550\times 10^{-9}~m}$
$E = 3.616\times 10^{-19}~J$
We can find the rate at which the sun emits photons:
$\frac{3.9\times 10^{26}~W}{3.616\times 10^{-19}~J} = 1.1\times 10^{45}~photons/s$
