Question 56984
The equation is s = n/2(a+t). 
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Here's how I understand the problem:
 s = {{{(n/2)(a+t)}}}. Solve for a
Multiply equation by 2, this gets rid of the denominator:
2s = n(a + t)
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Divide equation by n:
{{{2s/n}}} = a + t 
Subtract t from both sides:
{{{(2s/n)- t}}} = a
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Not sure how you got the answer that you gave.