Question 1087015
{{{a[n]=a[1]+(n-1)*d}}}
So,
{{{a[2]=a[1]+(2-1)*d=-18}}}
1.{{{a[1]+d=-18}}}
and
{{{a[5]=a[1]+4d=144}}}
2.{{{a[1]+4d=144}}}
Subtract 1 from 2,
{{{a[1]+4d-a[1]-d=144-(-18)}}
{{{3d=162}}}
{{{d=54}}}
Then from 1,
{{{a[1]+54=-18}}}
{{{a[1]=-72}}}
So,
{{{a[n]=-72+(n-1)54}}}
{{{a[n]=-72+54n-54}}}
{{{highlight(a[n]=54n-126)}}}