Start by looking at the equation
CaCO3 --.> CaO + CO2
We can see that one reactant decomposes to give 2 products.
10 g od CaCO3 produced 5.6g of CaO. Therefore the remainder of the mass must have been made into CO2
Mass of CO2 is then 10 - 5.6 = 4.4g
Please note this method will only work if there is only one reactant. If there is more than one reactant, you have to look at the molar ratios, if you need this explained, just send me a message.