How many different combinations are possible if a number cube is tossed three times?

It would be 6 x 6 x 6, since the number cube has six sides and it was thrown 3 times. 

If you toss it the first time there are 6 outcomes 

For each of these outcomes, the second toss could yield another 6 outcomes - total 6 x 6 = 6² outcomes 

For each of the these outcomes, the third toss could yield another 6 outcome - total = 6² x 6 = 6³ outcomes. 

Extending this pattern you see that for x tosses, the total number of outcomes is 6^x.
That's what I got, hope it helps you.