It is easier to find the chance that Harrydoes notpick a yellow ball and subtract it from 1.

There are 4 non-yellow and 2 yellow. Hence there is a that he does not pick a yellow ball first time around.

Second time there are now 5 balls, 3 of which are not yellow which means there is a he does not pick a yellow ball.

Because this is an and scenario we multiply:

Subtract this from 1 to find the chance of at least one yellow