Need help understanding why I'm getting the right answer for C and H, but not O:

A compound contains only carbon, hydrogen, and oxygen. Combustion of 11.75 mg of the compound yields 17.61 mg CO2 and 4.81 mg H2O. The molar mass of the compound is 176.1 g/mol. What are the empirical and molecular formulas of the compound? (Type your answer using the format CO2 for CO2.)

I called the compound X. So,

(11.75 mg X) / (171600mg/mol) - (17

Question

Need help understanding why I'm getting the right answer for C and H, but not O: 

A compound contains only carbon, hydrogen, and oxygen. Combustion of 11.75 mg of the compound yields 17.61 mg CO2 and 4.81 mg H2O. The molar mass of the compound is 176.1 g/mol. What are the empirical and molecular formulas of the compound? (Type your answer using the format CO2 for CO2.)

I called the compound X. So,

(11.75 mg X) / (171600mg/mol) -> (17