Assuming the number cube is a die numbered 1-6,
The probability of flipping a coin and getting heads and the probability of rolling a 3 on the die are independent of each other.
-There are two sides of a coin, heads and tails. You have two possible results from the coin - heads, or tails. You have a 1 in 2 probability of getting either.
-There are six sides of a die. There are six possible results from the die - 1, 2, 3, 4, 5, and 6. You have a 1 in 6 probability of getting any of the results.
The probability of both happening (and) can be determined by multiplying the two probabilities.
Hope this helps!
-refrac532