Answer
The light left the sun $~~8.4~minutes~~$ earlier.
Work Step by Step
During a full moon, the moon is on the opposite of the Earth from the sun. The light must travel from the sun to the moon, and then back to the Earth.
We can find the total travel distance:
$d = 1.5\times 10^8~km+3.8\times 10^5~km+3.8\times 10^5~km$
$d = 1.5076\times 10^8~km$
We can find the time it takes light to travel this distance:
$t = \frac{d}{c}$
$t = \frac{1.5076\times 10^{11}~m}{3.0\times 10^8~m/s}$
$t = 502.5~s$
$t = 8.4~minutes$
The light left the sun $~~8.4~minutes~~$ earlier.
