Question 936448
{{{a[n]=a[1]+d(n-1)}}}
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{{{a[15]=a[1]+14d=129}}}
{{{a[8]=a[1]+7d=22}}}
Subtract the two values,
{{{a[1]+14d-a[1]-7d=129-22}}}
{{{7d=107}}}
{{{d=107/7}}}
Then use either equation,
{{{a[1]+7(107/7)=22}}}
{{{a[1]+107=22}}}
{{{a[1]=-85}}}