I'm not great at these kinds of problems, but I always like to give 'em a try!!!.........
Let A be the probabilty of one side showing "red".......and let B be the probabilty that both sides are red.
We already know that the double-sided white card doesn't matter, since we already have a card showing "red."
So we have
(Probability of Two sides Red l One side is Red) = P(A and B) / P(A)
P(BlA) = [(1/2) * (1/2)] / (1/2) = (1/2)
Another way of looking at this is that we either have the double-sided red card, or we don't !!!
That's my two cents worth, anyway !!!