Ineed to know how to indicate this sum

Put n = 1

3 - 20 = -17

Put n = 2 ( incremented)

6 - 20 = -14

Put n = 3 ( incremented)

9 - 20  = -11

From the given values deduce a general recurrence relation. The series follow a pattern which is arithmetic sequence pattern.

ao = -17
d = -14 - (-17) = 3
 

S = n/2 (2ao + d(n-1))

n = 20 -1 + 1 = 20 (as you start from 1 not 2)

S = 20/2( 2 * -17 + 3(20-1)) = 230