I think the problem should say
A marble is chosen at random from the urn and it is noted that it is NOT one of the black marbles, what is the probability that it is yellow?
You can use either method that I used in my reply to Chiluya in response to an earlier question.
Since you know the marble you drew is not black this is the same as drawing a marble from an urn containing 10 white and 5 yellow marbles.
Let A be the event of drawing a yellow marble and B be the event of not drawing a black marble.
I hope this helps,