S=n/2 [a+(n-1)d]
make a subject

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

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

option (b) is correct

step-by-step explanation:

to find the solution of this system, we need to convert it into reduced row-echelon form. we have,

after applying row operations r2 -> r2/2 and r3 -> r3/3, we get,