The probability of getting a 3 in a single roll is 1/6. The probabilty of not getting a 3 in a single roll is 5/6.

a) The probability of 'no 3' is (5/6)^4. You can get 'no 3' in only 1 way. So the probabilty is (1) (5/6)^4 = ......

b) The probability of 'exactly three 3's' is (1/6)^3 (5/6). You can get 'three 3's' in four different ways:

3, 3, 3, x

3, 3, x, 3

3, x, 3, 3

x, 3, 3, 3.

So the probability is (4) (1/6)^3 (5/6) = ......