area of a rectangle with a semicircle on top  

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You'll want to find the area of the rectangle using base*height or length*width.  And then, the area of the half circle by taking the length of the side it's attached to as its diameter.  So I'll call the length of this side x, then the radius is (1/2)x, and the area of the full circle, A = pi*r^2 = pi*((1/2)x)^2 = (pi/4)*x^2, and you want only half, so the half circle area is (pi/8)*x^2.  Combine these to get the area of the figure:length*width + (pi/8)*x^2   (where x is either the length or the width, whichever the half circle is on)