At the start, 6 out of 26 marbles in the bag are white. So your
chance of drawing a white one is just that: 6/26.
Let's say you did that. Now what are your chances of drawing another
white one? There are 25 marbles left in the bag and 5 are white. So
the chance of drawing a white one in this situation is 5/25.
Since the two drawings are independent of each other, you just
multiply the probabilities together to get your answer.
Hope this helps,
Paul and Sue