The heights of the triangles (for there are two:
http://i34.tinypic.com/2rcsprd.png) are
{h \over 2}+r and
r - {h \over 2}, where r is the radius of the circle.
b is fixed based on h, according to the formula
b = sqrt{-{h^2 \over 4} + r^2}*2 (this follows from the formula for the circle).
The formula for the surface of the rectangle is
sqrt{-{h^2 \over 4} + r^2}*2*h, and for the triangles
{({{h \over 2}+r}) * sqrt{-{h^2 \over 4} + r^2}*2} \over 2 and
{(r - {h \over 2}) * sqrt{-{h^2 \over 4} + r^2}*2} \over 2.
The problem therefore reduces to finding the values of h for
sqrt{-{h^2 \over 4} + r^2}*2*h = {{({{h \over 2}+r}) * sqrt{-{h^2 \over 4} + r^2}*2} \over 2} on the one hand, and
sqrt{-{h^2 \over 4} + r^2}*2*h = {{(r - {h \over 2}) * sqrt{-{h^2 \over 4} + r^2}*2} \over 2} on the other.
(Hint: they will be the same.)
Since this is your homework, I suggest you solve those.