so hmm if you see the picture below, notice, the radius is just half the diameter of the semi-circle, the diameter is 16, so the radius is 8
the perimeter of the figure is how many units will be required for the fence, since the fence will be enclosing the garden
so... 25 + 16 + 25 + [ circumference of the semi-circle ]
recall the circumference of a circle is 2 π r, r = radius
so the circumference of a semi-circle, will be half that, or just πr