The perimeter of a rectangle is 2·l + 2·w. Since the width is covered by the semi-circles on each side, you do not add that into your equation, so just 2·l
Since two semi-circles are the same as one full circle, you can just add the equation for the perimeter of a regular circle into your equation, which is 2πr.
Therefore, P=2·l + 2πr
If you have any more questions just ask.