Answer
$7.3\times 10^{18}photons/s$
Work Step by Step
We know that
$\frac{n}{t}=\frac{P}{hf}$
We plug in the known values to obtain:
$\frac{n}{t}=\frac{2.5W}{(6.63\times 10^{-34}J.s)(5.2\times 10^{14}Hz)}$
$\frac{n}{t}=7.3\times 10^{18}photons/s$