Hi!
1. Assume that these percentages are the mass.
2. For each of Fe and O, divide the mass by the Ar (relative atomic mass).
Cl: 38.77 / 35.5 = 1.09211 moles
O: 61.23 / 16 = 3.82688 moles
3. Work out which mole value is smallest, then divide the bigger mole value by the smaller one.
1.09 is smaller than 3.83, so your empirical formula is 1.09/1.09 = 1 atom of Cl, and 3.83/1.09 = 3.51 atoms of oxygen.
4. Ensure your atom values are whole numbers.
3.51 is not a whole number, so multiply both 1 and 3.51 by 2.
3.51 x 2 = 7.02. This is close enough to a whole number that no further multiplication is required.
5. Write out your empirical formula.
Cl2O7
Hope that helps!