17

Σ (15 - 9n)

η = 4

17

Σ (15 - 9n) = - 1113

η = 4

nth term

aₙ = 15 - 9n

a₄ = 15-9*4 = 15-36 = - 21

a₁₇ = 15 - 9 (17) = - 138

Sum;

S = (a₁ + aₙ) * n/2

n = 14

a₁ = a₄ = - 21

aₙ = a₁₇ = - 138

S = (-21 - 138) * 14/2

S = - 1113

-1113

Step-by-step explanation:

You apparently want the sum of the 14-term arithmetic sequence ...

-21, - 30, - 39, ..., - 138

The average term is ...

(-21 - 138) / 2 = - 79.5

so the sum of the 14 terms is ...

(14) (-79.5) = - 1113