Find the sum of an arithmetic sequence!!

[ellaaaaaa]

I need help with this problem:

Find the sum of the arithmetic sequence given below.

-17, -14, -11, ..., 55

My working is attached in a photo, i'm unsure where I go wrong.. The correct answer I am supposed to get according to the book is 475

 

[Deleted member 4993]

I need help with this problem:

Find the sum of the arithmetic sequence given below.

-17, -14, -11, ..., 55

My working is attached in a photo, i'm unsure where I go wrong.. The correct answer I am supposed to get according to the book is 475

Everything looks good except the last line

answer = 25/2 * 38 = 25 * 19

The answer must be less than 25*20 (=500)

do the multiplication again!!