Ok, so the diameter of the circles is 6 because is the half of the rectangle,
the radius is half the diameter so is 3
the height of the rectangle is equal to the diameter of the circle so is 6
the shaded area is the total area of the rectangle minus the area of the two circles
then
[tex]\begin{gathered} (12\times6-2(3^2\pi))cm^2 \\ (72-18\pi)cm^2\approx15.5^{}cm^2 \end{gathered}[/tex]