In an arithmetic series the 3rd term is -8 and 12th term is -35. Determine the number of terms in the series if its sum is -100

Answer 1

a+2d = -8
a+11d = -35
so, 9d = -27
Now you can find a and d, and then solve
n/2 (2a+(n-1)d) = -100
n = 8