Calculating electron affinity from Born-Harber cycle

I was working through a thermodynamics question and asked to draw the born-harber cycle for Barium Chloride, which I managed to do. However, I cant seem to understand how to calculate the electron affinity.

I understand that you multiply the chloride affinity and atomisation by two so the unknown becomes 2x but from the mark scheme its unclear to me why you equal everything to zero. You arnt given the dissociation either?

I have attached the question and mark scheme, explanation would be appreciated