The probability you get a black marble on the first selection is 2/3 since we have 2 black marbles out of 2+1 = 3 total.
We put the marble back and then we have 2/3 as the probability of selecting another black marble on the second try. Nothing has changed because we put the marble back. That means the events are independent.
So we get (2/3)*(2/3) = 4/9 as the probability of selecting 2 black marbles in a row (with replacement).
