I am correcting a test but need the formulas

[Petie]

Are we assuming that the planet and Earth don't move ? Because things are about to get mathematically insane if we factor in rotation around the planets' respective stars.

EDIT - If not, what Petie said.

Slight variations in distance based on the position of the planets in question would have a negligible impact on the calculation given the vast distance between them to begin with. Plus, there's not enough information given to factor it in even if you wanted to - we don't know the radius of the orbit or where each planet starts on their orbits to determine how much more/less the light would need to travel.