Question 977865
What do you mean by "the number of the sequence is 17"? 
It's not the first number.
It's not the second number.
Is it the common difference between values? 
If so, then
{{{a[2]=a[1]+d}}}
{{{3=a[1]+17}}}
{{{a[1]=3-17}}}
{{{a[1]=-14}}}