X^2y^2Z^2? Consider 6 identical boxes each with 3 distinct items, x, y, and z. First, pick 2 x's from all the boxes. There are 6*5/2 ways. (/2, for the picking order doesn't matter.) Next, pick 2 y's from all the unpicked boxes. 4*3/2 ways. Finally, pick 2 z's from all the unpicked boxes. 1 way. 90 ways. The answer is 90.