struggleing on this question..please help?

Question

The standard Gibbs free energy change for hydrolysis of fructose-1,6- bisphosphate (FBP) to fructose-6- phosphate (F-6-P) and pi is -16.7 kj/mol:

FBP + H20 -----------> F-6-P + Pi

The standard Gibbs free energy change for ATP hydrolysis is -30.5 kj/mol:

ATP + H2O -----------> ADP + Pi

The standard Gibbs free energy change for the phosphofructokinase reaction shown below, is which of the following.
ATP + F-6-P-------------->ADP + FBP

(a) -47.2 kj/mol
(b) +13.8 kj/mol
(c) -13.8 kj/mol
(d) none of the above

Does anyone which is the correct answer?
Thankyou,xx

Answer 1

C. To get the equation for which you want delFo, you have to REVERSE the 1st equation and ADD it to the second. Thus, you have to reverse the SIGN of the first delFo and add it to the second.