##4.2xx10^(-5)## light years
Light-years is not an adequate measure for distances inside the solar system. Symply a light-year is a humongously large measure of distance.
The distance between Earth and Mars is at the most 400 million km and could be as small as 55 million km.
https://en.wikipedia.org/wiki/Mars
Even with the larger distance we would obtain:
##(400xx10^6 km )/(300000 km//s)~1333s ## which is about 22 minutes. So Mars can be at the most 22 light-minutes from .
Neverthless here is your answer:
Just divide the time in seconds light takes between Earth and Mars by the number of seconds within one year:
##1333/(365.25xx24xx60xx60)=4.2xx10^(-5)## light years