Question 972260
If it's an arithmetic sequence then,
{{{d=a[2]-a[1]=3-17=-14}}}
So,
{{{a[3]=3-14}}}
{{{a[3]=-11}}}
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If it's a geometric series then,
{{{r=a[2]/a[1]=3/17}}}
So,
{{{a[3]/a[2]=3/17}}}
{{{a[3]=3(3/17)}}}
{{{a[3]=9/17}}}