They are given the equations provided, and through cancelation (and in some cases) manipulation of the equations and G given for equation, arrive at the needed equation and G for new equation.
C + O2 + Ca + 1/2 O2 --> CaCO3
If memory serves, then use the dG for equation against values found in table (back of Chem text) for an explicit value for CaCO3.
If you need more help, feel free... but this should give you enough to solve the problem on your own. We are prevented from providing the solution to you.
But you need to manipulate the equations and arrive at the etropy through addition / subtraction / multiplication / division of the dG.
Again, until I can get home and pull out text books, I hesitate to go further.
(Its been awhile).