The sum of the first 21 terms of the progression,-18,-15,-12,is (solution)
a = -18
d = 3
sum(21) = (21/2)(2a + 20d)
= (21/2)(-36 + 60) = .....
looks like arithmetic sequence with a = -18 and d = +3
Tn = -18 + (n-1) * 3
You can look up the sum of the first 21 terms of an arithmetic sequence