please explain in steps

Find the sum of the first 28 terms of the following sequence:

-7, -4, -1 . . .
938 1876
134 588

Question

please explain in steps 

Find the sum of the first 28 terms of the following sequence: 

-7, -4, -1 . . .
	938		1876
	134		588

solomonzelman · Answer

a1=-7
d=3

$\large\color{blue}{ \rm a_{n}=a_1+d(n-1)}$
$\large\color{blue}{ \rm a_{28}=-7+3(28-1)}$

solomonzelman · Answer

after you find the a28,

$\large\color{blue}{ \rm (a_1+a_{28}) \times (1/2) \times 28}$

starryknight · Answer

i tried doing it and got 1330  using s(n)=(2a+(n-1)d)

starryknight · Answer

the guy that helped me told me to use that

starryknight · Answer

28/2(14+(28-1)(-3)

starryknight · Answer

is that how you do it

solomonzelman · Answer

I get 74 for a28. and S28=1036

solomonzelman · Answer

fisrt you have to find a28, *as I told you) and then plug in (a1 and a28) into the sum formula.