In the given arithmetic series, a = 24, d =- 3.
Let us find n such that Sn = -351
Now, Sn = n/2[(2a + (n-1)d]
– 351 = n/2[(48 + (n-1)x(-3)]
on solving we get, n2 - 17n -234 = 0
(n - 26h)(n + 9) = 0
n = 26 or n = - 9
Here n, being the number of terms needed, cannot be negative
Thus, 26 terms are needed to get the sum -351.
