Question 463762
{{{a[8] = a[3] + (8-3)d = a[3] + 5d = 55}}}

and

{{{a[10] = a[3] + (10-3)d = a[3] + 7d = 77}}}

Subtracting the top equation from the bottom equation, we get  2d = 22, or d = 11.
Hence {{{a[3] + 5*11 = 55}}}, or {{{a[3] = 0}}}.