If so, start by factoring the numerator. Then cancel any common term in the numerator and denominator. If you end up with no denominator, then use the original denominator. Set it equal to zero and solve for x. That value of x will be the restriction.
There are 6 sides on a dice. Every time you roll you you will have one of those sides. So it is a 1 in 6 chance of you getting 4, meaning that the answer to this problem is 1/6