again i'm having trouble understanding this, i'm confused on what formula to use. :(

Calculate ΔG° (298 K) for the following reactions from ΔfG° data:

a)CaO (s) + H2O (l) ---> Ca(OH)2 (s)

b)2C2H5OH( (l) + 602 ---> 4CO2 (g) + 6H2O (l)

c)2HNO3 (l) + 2HCL (g) ---> Cl2 (g) + 2NO2 (g) + 2H2O (l)

ΔfG° (kJ mol-1)

CaO(s) –604

H2O(l) –237

Ca(OH)2(s) –897

C2H5OH(l) –175

CO2(g) –394

HNO3(l) –80

HCl(g) –95

NO2(g) +52