This is the formula for the sum of the first n terms of an arithmetic series. I suggest you consult your textbook or class notes for the derivation (or Google it).
If you're attempting to apply this formula in a specific question, it would help if you actually posted that question. We are not mind readers.
Sorry it was solve for a and I just can't see to figure out how they achieved the answer they got. How would you approach this. In order for me to understand I would need to see how you reached your answer.
Anyone who is up for the challenge. I would greatly appreciate it.