Answer
$5.7\times 10^8m$
Work Step by Step
We know that
$\Delta t=\frac{E}{P}$
We plug in the known values to obtain:
$\Delta t=\frac{9.5mJ}{5.0mW}=1.9s$
Now we can find the required the length as
$L=c\Delta t$
We plug in the known values to obtain:
$L=(3.00\times 10^8)(1.9)=5.7\times 10^8m$
