An oil spill is 6 cubic meters, and the thickness is 0,002 cubic meters.

How many kilometers is it?

I would expect the problem to say something like this:

An oil spill covers an area of 6 cubic meters, and the thickness is 0,002

meters.

How many

cubic meters is it?

A thickness can't be measured in cubic meters (a unit of volume), and there is no length in kilometers that can be determined.

If the problem is as I suggest, then you can find the volume using the formula V = Bh, where B is the area of the base and h is the "height" (thickness, in this case).