Since this is an arithmetic sequence, you can conclude that the addition to the number is constant.
Let x = the number asked
k = constant number added to the sequence
7 + k = x
x + k = 17
Solve the system of equation:
7 + k + k = 17
7 + 2k = 17
2k = 10
k = 5
Therefore, the missing number is 12 (7 + 5).