Question 880739
General term, {{{a+(n-1)d}}}.  First term is a.  Common difference is d.  n is the index of any term.


First Two Terms Sum:
{{{a+(1-1)d+a+(2-1)d=16}}}
{{{a+a+d=16}}}
{{{highlight_green(2a+d=16)}}}


Second and Third Terms Sum:
{{{a+(2-1)d+a+(3-1)d=28}}}
{{{a+d+a+2d=28}}}
{{{highlight_green(2a+3d=28)}}}


Starting with Elimination Method would be best because both equations have a 2a term.
{{{(2a+3d)-(2a+d)=28-16}}}
{{{2d=12}}}
{{{d=6}}}
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The simplified term one and two sum equation can be chosen for finding a.
{{{2a+d=16}}}
{{{2a=16-d}}}
{{{a=(16-d)/2}}}
Substitute for d:
{{{a=(16-6)/2}}}
{{{a=5}}}
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The general term is now more specifically, {{{highlight(5+(n-1)*6)}}}.
First three terms:   5, 11, 16.