You need to convert meters to light years, so divide the distance by the distance light travels in a year, and you have light years it takes for light to get there.
For part b, the answer they're looking for is that using light years requires the use of smaller exponents, so is supposedly more easy to grasp. The light year really isn't used for distances beyond our own small corner of the galaxy... for things on the other side of the galaxy, and anything that's not in our own galaxy, the most common unit of measurement is the parsec (intergalactic distances are typically given in Mpc, megaparsecs).