Lets take a real arithmetic progression:
1, 5, 9 , 13, 17
As you can see any term of the above arithmetic progression is 4 greater than the previous term. 1+4 = 5... 5+4 = 9.... so on
So if we draw parallels with the question above here
c = 4. Then we can write the above series taking the middle term 9 into account:
9-2*4, 9-4, 9 , 9+4, 9+2*4
a-2*c, a-c , a , a+c, a+2*c
Thats how Sandy derives the a+2c and a-2c terms
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