What is the empirical formula C12H6O3?

The principle for this kind of problem is to find the highest integer that is an integral factor of all the subscripts, then divide the subscripts by that integer. In this instance, 3 is the highest integer, since 15/3 = 5, a Prime number, and 5 is not an integral factor of 9 or 3. Therefore, the empirical formula would be C3H5O.