In order to derive the base of a triangle from its area, you need its height as well.
In fact, if we solve the area formula for the base, we have
[tex]A=\dfrac{bh}{2} \iff 2A = bh \iff b=\dfrac{2a}{h}[/tex]
So, the base length would be
[tex]b=\dfrac{2\cdot 28.7}{h} = \dfrac{57.4}{h}[/tex]
where h is the height relative to the base you're interested in.
