Answer :
Check the picture below.
we know the area is 1586, so
[tex]\stackrel{\textit{area of a trapezoid}}{A=\cfrac{h(a+b)}{2}}\implies 1586=\cfrac{26(84+b)}{2}\implies 1586=13(84+b) \\\\\\ \cfrac{1586}{13}=84+b\implies 122=84+b\implies 38=b[/tex]

Check the picture below.
we know the area is 1586, so
[tex]\stackrel{\textit{area of a trapezoid}}{A=\cfrac{h(a+b)}{2}}\implies 1586=\cfrac{26(84+b)}{2}\implies 1586=13(84+b) \\\\\\ \cfrac{1586}{13}=84+b\implies 122=84+b\implies 38=b[/tex]