Answer
$1.0\times10^1ns$
Work Step by Step
The time we seek is the time for the EM wave to travel the 3-m distance.
$$\Delta t= \frac{3m}{3.00\times10^8m/s}\frac{10^9ns}{s}= 1.0\times10^1ns$$
Incidentally, a rule of thumb for the speed of light is that it travels at about a foot per nanosecond. Three meters is about 10 feet, so a rough estimate agrees with this answer.
