The probability of even a is x, and the probability of even b is y, if the two evens are independent, which of these conditions must be true?

I think you meant to write "event" instead of "even".

P (A) = x

P (B) = y

P (A and B) = P (A) * P (B)

P (A and B) = x*y

you multiply the two probability values. This rule only works because the two events are independent.