How to interpret conditional probability from complex questions?

As you are finding conditional probability, you need to first check the given condition. The given condition is that you have drawn one candie each from the old and the new bag and one of the candies is yellow and one of the candies is green. Now there are two possibilities - $i)$ the yellow candie came from the old bag and the green candie came from the new bag $ii)$ the yellow candie came from the new bag and the green candie came from the old bag.

As per the question, you need to find the conditional probability that the yellow candie came from the old bag (and that the green candie came from the new bag). So it should be,

$ \small P(\text {yellow from old bag}| \text{one green and one yellow}) = \dfrac{P(A)}{P(A) + P(B)}$

where,

$ \small P(A) = P(\text{yellow from old bag}) \cdot P(\text{ green from new bag})$

$ \small P(B) = P(\text{green from old bag}) \cdot P(\text{ yellow from new bag})$