The relative formula mass (Mr) of a compound is the sum of the relative atomic masses (Ar) of all the atoms in the formula, in the numbers shown. Mr has no units. Use these Ar values: H = 1, C = 12, O = 16, Mg = 24, Ca = 40.
For CO2 you add the Ar of one carbon atom and two oxygen atoms: 12 + 16 + 16 = 44. For MgO the Mr is 24 + 16 = 40. For CaCO3 the Mr is 40 + 12 + (3 × 16) = 100.
In a balanced equation, the sum of the Mr of the reactants in the quantities shown equals the sum of the Mr of the products. For 2Mg + O2 → 2MgO the reactants give (2 × 24) + 32 = 80 and the products give 2 × 40 = 80, so the total mass is conserved.
You can also find the percentage by mass of an element in a compound. Divide the total Ar of that element in the formula by the relative formula mass of the compound, then multiply by 100. In MgO, magnesium is 24 ÷ 40 × 100 = 60%, so oxygen is 40%. In CaCO3, calcium is 40 ÷ 100 × 100 = 40%.