Question 969239
{{{a[n]=a[1]+(n-1)d}}}
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{{{a[17]=a[1]+(17-1)d=X}}}
{{{a[1]+16d=X}}}
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{{{a[2]=a[1]+d=3}}}

Subtracting this from the first equation,
{{{a[1]+16d-a[1]-d=X-3}}}
{{{15d=X-3}}}
{{{d=(X-3)/15}}}
Then,
{{{a[1]+(X-3)/15=3}}}
{{{a[1]=3-(X-3)/15}}}
{{{a[1]=45/15-(X-3)/15}}}
{{{a[1]=(45-X+3)/15}}}
{{{highlight(a[1]=(48-X)/15)}}}