You are going to roll two dice. let the variable x = the sum of the numbers rolled. what is the probability that x = 9 ?

The  first cube can land in any one of  6  ways.
The  second cube can land in any one of  6  ways.
Total number of ways that  2 dice can land = (6 x 6) = 36 ways.

For any of these ways, the sum of the numbers rolled is  9 :

3, 6
4, 5
5, 4
6, 3

There are  4  ways to roll a 9, out of a total of  36  ways that
the dice can land.  So the probability of rolling a  9  is

       4 / 36  =  1 / 9  =  11-1/9 percent

Element 17 is chlorine it is a group 7 element(halogen) and belongs to period 3 of the periodic table