When two dice are rolling, the total space of events has 36 elements. 
These elements are the pairs (red,green),  where "red" and "green" are integer numbers from 1 to 6 inclusive.


These 36 elements of the space of events have equal probability of    each.


Of these 36 elements, exactly 6 (six) have the sum of 7:


    red     green

     1        6

     2        5

     3        4

     4        3

     5        2

     6        1


and only one pair  (red,green) = (2,5)  is favorable.


Therefore, the answer to the problem's question is 


    probability  P  =  = .    ANSWER




I made this simple calculation using the full space of events.



But you can easily get the same answer using the REDUCED space of events.

It consists of 6 listed elements, and only one of them is favorable;  so


    P = ,   


giving the same answer.