an= -77, n=16, d=-5 find the sum of arithimetic

Question

I know that the answer is -632. But I do not understand. I do not know how to fiqure out the a1

Answer 1

a₁₆ = a₁ + (16 - 1)d

-77 = a₁ + 15(-5)

Can you now solve for a₁?

Then use S₁₆ = 16/2(a₁ + (-77))