I would choose to have a higher initial rate of zombies with a slower growth rate because (although it would be difficult in the beginning) i would have more time to prepare for the zombies.
I would chose more initial zombies, but a slower growth rate because if their growth rate is slow then you have time to find weapons and build you a shelter then as they grow stronger you grow stronger so you have a better chance of surviving
Just multiply how many miles in one light year (provided in question which is 1 LY = 5.88*10^12 miles) and multiply that by 13.4 and you should get 7.88*10^13 miles for the answer.