I think, the solution of @yg92 is correct.
just check it.
since in the question they have mentioned that one side of the card is white and they are willing to know or the favourable case is white on both the side.
Hence, the numerator should be (1/3) * 1, because 1/3 is for choosing the card and 1 as we know that its both faces are white, hence probability of white on that card will be 1 and that is what we want.
And the denomenator will the total probability.