Question 1177512
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The problem does not disclose which of the three sides of the triangle is called "the base"; 

so I will assume that the base is the horizontal segment AB.



Then the length of the base is  |8-a|  units, while the height is 4 - (-1) = 5 units.


We find "a" from this absolute value equation


    | 8-a | = 2*5,  


which is equivalent to


    | a-8 | = 10.



It has two solutions  a= 18  or  a= -2.      


<U>ANSWER</U>. Possible values of "a" are  -2  or  18.
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Solved, answered, explained and completed.