jmc

This triangle is isosceles, and so the altitude to the side with length 6 must hit that side at its midpoint. Thus our triangle is divided into two right triangles with hypotenuse $5$ and one side of length $3$. Thus each of these is a $3-4-5$ triangle, and each one has area $\frac{3\times 4}{2}=6$, for a total area of $\boxed{12}$.