Question 958597
{{{a[n]=a[0]+n*d}}}
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{{{a[7]=a[0]+7d=12}}}
{{{a[10]=a[0]+10d=-9}}}
Subtracting,
{{{a[0]+10d-a[0]-7d=-9-12}}}
{{{3d=-21}}}
{{{d=-7}}}
Then use either value for find {{{a[0]}}}.
{{{a[0]+7(-7)=12}}}
{{{a[0]-49=12}}}
{{{a[0]=61}}}
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{{{a[n]=61-7d}}}