Answer:

Thus a3 = a +(3–1)d = -13

a6 = a+(6–1)d = -4

Solving both equations,

d = 3 and a = -19

Sum of n terms = Sn = (n/2) * (2 a +(n-1)d))

S12 = (12/2) * (-38 + 33)

S12 = 6 * (-5) = -30

Step-by-step explanation:

correct me if I'm wrong.