it's an incomplete question
here is your example mate .
1. when rolling a dice there is a chance to get number between 1 to 6. is always happen.
2. when rolling a dice it is impossible to get number 7 because there are only 6 number .
3. when rolling a dice it may or may not be possible to get the number you want between 1 to 6.
hope my answer you
and answer is y(3x-4y)(3x-4y)...