please scale your image file to less than 600X600 in future. regarding your thermodynamics question, keep in mind that G is a state variable, therefore its value is independent of the pathway, hence u can simply sum the values up for the two values of the standard and non-standard G.
for A->C,
non-standard deltaG = +53.4kJ/mol
standard deltaG = -21.5kJ/mol
for C->A,
non-standard deltaG = -53.4kJ/mol
standard deltaG = +21.5kJ/mol
for (c) & (d), you may quote from the given data.