In 5.2 moles of C7H18 there are 7*5.2 moles of carbon which is 36.4.
This is because for every molecule C7H18 there will always be 7 carbons.
Remember that a mole is just a way of quantifying stuff
So if we have 6.02*10^23 molecules, we will have 7*6.02*10^23 carbon atoms.
Therefore, we just have the number of molecules in a different unit, the mole so its still 7 times greater.
Another example would be that there is 5.2*18 = 93.6 moles of H since there is 18 atoms of hydrogen per molecule of C7H18.