There are 36 outcomes when two dice are rolled.
To get a 6 total, the outcomes are 1-5, 2-4, 3-3, 4-2, 5-1.
To get doubles, the outcomes are 1-1, 2-2, 3-3, 4-4, 5-5, 6-6.
The combined set of outcomes is {1-1, 1-5, 2-2, 2-4, 3-3, 4-2, 4-4, 5-1, 5-5, 6-6}. That’s 10/36=5/18. The probability is 5/18.