The sum of the first 21 terms of the progression ,_18,_15,_12,---is
_ and - are not the same!
Anyway, it should be clear that for this AP,
a = -18
d = 3
So, using your trusty formula,
S21 = 21/2 (2a+20d)
Now plug and chug.
Anyway, it should be clear that for this AP,
a = -18
d = 3
So, using your trusty formula,
S21 = 21/2 (2a+20d)
Now plug and chug.