As you know, the Law of Conservation of Mass states that for any chemical reaction that takes place in an isolated system, the total mass of the reactants must be equal to the total mass of the products.
Simply put, mass cannot be created or destroyed. The number of atoms that act as reactants will be equal to the number of atoms that end up as products.
In your case, you know that you must react carbon with excess oxygen. The product of this reaction will be a compound that contains carbon and oxygen.
More specifically, all of the carbon atoms that actually react with oxygen molecules will now be a part of the product.
The same can be said for the oxygen molecules. Every molecule of oxygen that reacts with an atom of carbon will be present in the product.
So, if you start with
#m_"product" = m_"carbon" + m_"oxygen"#
#m_"oxygen" = "88 g" - "24 g" = color(green)("64 g oxygen")#
The difference between the mass of the product and the mass of the carbon will represent the mass of oxygen that must have reacted with the carbon.