You have assumed that the molecues fit together perfectly. This is only a division problem.
= Number of Molecules
Where are you struggling, this setup or the unit conversion?
This is the problem:
"Suppose 300 cubic meters of oil is spilled into the ocean. Find the area of the resulting slick, assuming that it is one molecule thick and that each molecule occupies a cube 0.50 µm on a side."
The volume of the spill is
We know the volume of the spill and we know that the spill is one molecule thick, so
as TKHunny said.
Now get the units on top and bottom to be similar:
Always always always keep track of your units!