My guess is:
9. Each side adds up the Defense numbers of all units in the Melee to arrive at their total Defense.
10. Each side adds up all HE attack FP of all units in the Melee to arrive at their total FP.
By looking at Rules/6/Crewed Weapons the 57 ATG has an HE attack of 3 and a Defense of 4.
Xavier is correct. Use HE attack FP of 3 and the units Defense number of 4.
Another example with HMG, FP of 3 and a Defense of 3.