How many terms of the A.P. – 6, -11/2 , – 5, … are needed to give the sum –25?

How many terms of the A.P. – 6, -11/2 , – 5, … are needed to give the sum –25?

Answer 

Let the sum of n terms of the given A.P. be –25.
use the formula of Sn in which n = number of terms, a = first term, and d = common difference
Here, a = –6