7 odd terms:

7 even terms:

but consider:

each bracketed expression is teh difference between two consecutive terms of the AP. There are 7 of them in total.

But you KNOW for an AP that the difference between consectuve terms is just d (the common difference)

That gives you d directly.

Then , you can use your equation to work out and then get (assuming your equation is right, i didn't check it)