You'll need to multiply. This is because they both should happen. Addition is used when two events can't happen at the same time and you want to know the probability either occurs.
The probability of A hitting a target is 2/3. And probability of B hitting the same target is 4/7. So, what is the probability that neither of them will hit the target?
i did like !P(a)=1/3 and !p(b) is 4/7 so now i have multiply them or add them to get the answer