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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