(d=8), (n=19), (S_n=1786)

[ S_n = \dfrac{n}{2}\bigl[2a_1+(n-1)d\bigr] ] [ 1786 = \dfrac{19}{2}\bigl[2a_1+18\cdot8\bigr] ] [ 3572 = 19(2a_1+144) ] [ 188 = 2a_1+144 \implies 2a_1=44 \implies a_1=22 ]

Answer: (a_1=22)