Question 969779
We need more information than that.
Are there any terms before 3?
If not, then 3 is the first term.
Is 3 the second term, and 17 the third term?
 
If it is an arithmetic sequence, then each term is the one before plus a fixed number, {{{d}}} .
That number {{{d}}} is called the common difference, because it is the difference between any two consecutive terms.
So, term number {{{n+1}}} , {{{a[n+1]}}} ,
is related to the previous term,
term number {{{n}}}={{{a[n]}}},
by {{{a[n+1]=a[n]+d}}}<--->{{{d=a[n+1]-a[n]}}}
for all natural number (counting number) values of {{{n}}} .
If 3 and 17 are the 2nd and 3rd terms respectively,
then the common difference between consecutive terms is
{{{d-17-3=14}}} .
Each term is {{{14}}} more than the one before.
{{{a[2]=3}}} , {{{a[3]=17}}} , {{{d=14}}} , and {{{a[1]}}} is the first term that we want to find.
From {{{a[n+1]=a[n]+d}}} , for {{{n=1}}} , we get
{{{a[1+1]=a[1]+d}}}--->{{{a[2]=a[1]+14}}}--->{{{3=a[1]+14}}}
so {{{a[1]+14=3}}}--->{{{a[1]=3-14}}}--->{{{a[1]=-11}}}