PLEASE SOMEONE HELP ME WITH THESE 3 SIMPLE QUESTIONS..PLEASE PLEASE PLEASE. ALGEBRA 2.. ARITHMIC SERIES, ECT.

I really need help! I am struggling to make straight A's & im in desperate need to figure out how to do this... so please please please take a little time out of your day to help me out.. I would really appreciate it... and DONT BOTHER WRITING SOMETHING IF IT DOESNT GIVE ME THE ANSWER! THX ;D

The sum of an arithmetic series is 1356 and the first term is -1. If the common difference is 5, how many terms are in the sequence?

Please show your work

The sum of a geometric series is -513. If the first term is -3 and the common ratio is -2, what is the final term in the sequence?

Please show your work again...

Evaluate: Show your work. This one you have to show work!
C=(18,12)

PLEASE PLEASE HELP ME... THANK YOU.

The equation for sum of arithmetic sequence = n/2(2*first term+(n-1)(d))

So n is what we need to find.

So n/2(2*(-1)+(n-1)(5)) = 1356
So n/2(-2+5n-5) = 1356
So n/2(5n-7) = 1356
So n(5n-7) = 2(1356) = 2712
So 5n^2 - 7n = 2712
So 5n^2 - 7n - 2712 = 0
So using the formula (-b+-sqrt(b^2-4(a)(c))/(2a)
(-(-7)+-sqrt((-7)^2-4(5)(-2712))/(2(5)…
(7+-sqrt(54289))/10
So n = 24
or n = -22.6 which isn't possible.

So n=24