2/3, simple conditional probability:
A: top card is white
B: bottom card is white
P(B|A) = P(A and B) / P(A)
P(A and B) = P(both top and bottom card is white) = 1/3 because it only happens if you pick the card that is white on both sides.
P(A) = 3/6 because there 6 sides total, 3 W and 3 B.
P(B|A) =(1/3) / (1/2) = 2/3.