Question

Why do you make it clear so that I can understand how to calculate

Answer 1

The sum of the first eight terms of an arithmetic progression is 220 if the third terms is 17.

Find the sum of the first six terms?

Answer 2

8/2 (2a+7d) = 220

a+2d = 17
a=3 and d=7
That makes the sum you want
S6 = 6/2 (2*3+5*7) = ____