How much CO2 gets free when combusting 6 kg n-heptane (C7H16)
For these types of problem, concept of mole is very handy. Here, you know the reaction stoichiometry, that means you have at least three information from that reaction equation. How?  

** For every mole heptane, you need 11 mole oxygen for complete combustion
** For every mole of heptane combustion, you will get 7 mol carbon di oxide and 8 mol water. 

So, now if you know the mole corresponding to 6 Kg heptane you can easily find out the amount of carbon di oxide produced in mole unit.
The relation is
$$n_{CO_2}=7 \times n_{Heptane}$$
$$n_{Heptane}=\frac{m_{Heptane}}{molecular~weight~of~heptane}$$
Now if you want to also calculate amount of carbon di oxide produced in mass unit or volume unit, you need to use the second equation or ideal gas law equation.