How do I derive for a series in sigma notation?

My arithmetic sequence is 14.25 + (n-1).25
so my series would be (n/2)(28.5 + (n-1).25)

The common difference is .25 and my first term is 14.25. The original linear equation for these sequences/series was (1/4)x + 14.

I understand sigma notation, and how to find partial sums and all that. I just don't get this specific question they're asking about deriving the formula for the series in sigma notation. Please help.

Question

How do I derive for a series in sigma notation?

My arithmetic sequence is 14.25 + (n-1).25
so my series would be (n/2)(28.5 + (n-1).25)

The common difference is .25 and my first term is 14.25. The original linear equation for these sequences/series was (1/4)x + 14.

I understand sigma notation, and how to find partial sums and all that. I just don't get this specific question they're asking about deriving the formula for the series in sigma notation. Please help.