A is the first roll of the dice, B is the second.
The question is to find the expected value of A given A + B = 7; E(A|A+B = 7)
Since A can be any number between 1 and 6, is this as simple as (1/6)*1 + (1/6)*2 + (1/6)*3 + (1/6)*4 + (1/6)*5 + (1/6)*6?
Or am I missing something?
Well, B can be 6 different things
It can be 1,2,3,4,5, or 6
Each of these only has one value of A that makes A+B=7, right?
A+B=7 can be rewritten as A=7-B, which shows that each possibility of B gives 1 A
If B is random, then so is A.
If A could be 6,5,4,3,2, or 1, then what is the average if it is randomly chosen?
TL;DR, your answer is correct, and I presume that the method you used to get there was too.