[tex] s = \dfrac n 2 (a + (n-1) d)[/tex]
[tex]\dfrac{2s}{n} = a + (n-1)d[/tex]
[tex]\dfrac{2s}{n} -(n+1)d = a[/tex]
We could expand this further but let's call it done.
Answer: a = 2s/n - (n+1)d
