Explain this to me please.

Suppose an urn contains 10 red balls and 7 white balls and you draw 3 without replacement. If at least one of the balls drawn is white what is the probability that the other two are red? Leave your answer either in terms of binomial coefcients or as an unrounded decimal. Hint: Set up a conditional probability P(A|B) where B is the event at least one of the three is white and A is the event that one is white, 2 are red