Question 1086752
{{{ S = (n/2)(a+l))  }}}    <<< Assuming this is what you intended for starting expression, same as S=(n/2)(a+l)
{{{ (n/2)(a+l) = S }}}      ( Rearranged left and right sides, since we're solving for n let's write it on left. )
{{{ n(a+l) = 2S}}}          ( Multiplied both sides by 2. )
{{{ n = 2S/(a+l) }}}         ( {{{ cross(Multiplied) }}} Divided both sides by a+l.  Done. ) 
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Your 2nd step above has an error:   S = (n/2)(a+l) becomes 2S = n(a+l)  after multiplying both sides by 2, not  2S=n(2a+2l).
  
{{{S  = (n/2)(a+l) }}}
{{{2S = (2n/2)(a+l)) }}}
{{{ 2S = cross(2)n/(cross(2)) * (a+l) }}}

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7/5/17 - I corrected my last two steps, which had an error.  Apologies for that.
7/8/17 - Meant to add that the domain restriction {{{ a+l <> 0 }}} must accompany the answer.