Answer:
3.
Step-by-step explanation:
If its an AS it well have a common difference, so 3rd term - 2nd term should = 2nd term - first term:-
29 + x - (28 - x) = 28 - x - (15 + x)
2x + 1 = 13 - 2x
4x = 12
x = 3.
The sequence is
15 + 3, 28-3, 29 + 3
= 18, 25, 32
which is arithmetic with common difference 7.
