You roll 2 number cubes whats the probability of it being 7

Total number of possible outcomes = 36

Total number with sum 7 =5 
{1, 6} {2, 5} {3, 4}, {5, 2}, {6, 1}

P(sum 7) = 5/36 

5/36

You have to know all of the combinations the dice can show:

11 12 13 14 15 16
21 22 23 24 25 26
31 32 33 34 35 36
.
.
61 62 63 64 65 66

6x6=36 possible combinations handing 11 possible numbers(2,4,5,6,7,8,9,10,11,12)
As you can see some combinations repeat themselves
We can obtain number seven rolling dice in these combinations:
16 61
25 52
34 43

The probability of obtaining 7 would be how many combinations give us a 7 divided by how many possible combinations there are

P(7)= 6/36=1/6