Your welcome and welcome to the forum!

]]>we have

this means we have 2 n's for equation1. With 2 n's we can get a1 ( the first term ) and d ( the distance between each term ). We have the 2 n's they are n =10 and n = 20.

Substituting into 1:

Clean em up:

Solving:

a1 = 3

d = 4

Can you finish up now?

]]>I am getting:

3, 7, 11, 15, 19, ...

]]>S= Sum]]>