Question 983042
Just taking a guess, you have common difference of 17 as "number of the sequence".  Term at index 2  is  3.  If common difference is 17, assuming going forward, then going backward to index 1, you subtract 17 from 3:


{{{A[1]=A[2]-17}}}
{{{A[1]=3-17}}}
{{{highlight_green(A[1]=-14)}}}