Answer Happy

I only need help with Part B. I DO NOT need help with Part
A (the answer is ~ -27 kJ/mol).
The answer to Part B is supposed to be 8 electrons, but I keep
not getting that answer. I am really confused so if you could
show the steps, that would be greatly appreciated!
5.51 The following reaction occurs in the cytochrome bof complex, a component of the electron transport chain of plant photosynthesis: cyt-b(red) + cyt-f(ox) = cyt-b(ox) + cyt-f(red) (a) Calculate the biological standard Gibbs energy of this reaction. . (b) The Gibbs energy for hydrolysis of ATP under conditions found in the chloroplast is –50 kJ moll and the synthesis of ATP by ATPase requires the transfer of four protons across the membrane. How many electrons must pass through the cytochrome bof complex to lead to the generation of a transmembrane proton gradient that is large enough to drive ATP synthesis in the chloroplast?