Question

S=n/2 [a+(n-1)d]make a subject​

Answer 1

$s = \dfrac n 2 (a + (n-1) d)$

$\dfrac{2s}{n} = a + (n-1)d$

$\dfrac{2s}{n} -(n+1)d = a$

We could expand this further but let's call it done.

Answer: a = 2s/n - (n+1)d