Answer
$P = 4.32\times 10^{-10}~W$
Work Step by Step
We can find the energy of each photon:
$E = \frac{h~c}{\lambda}$
$E = \frac{(6.626\times 10^{-34}~J~s)(3.0\times 10^8~m/s)}{460\times 10^{-9}~m}$
$E = 4.32\times 10^{-19}~J$
We can find the power of the flash:
$P = \frac{N~E}{t}$
$P = \frac{(10^8)(4.32\times 10^{-19}~J)}{0.10~s}$
$P = 4.32\times 10^{-10}~W$