You need to know the formulas for the circumference and area of a circle.
The circumference of a semicircle is
But, since there are two semicircles that make one whole circle, we have:
But, the radius of the circle is w/2.
The perimeter of the rectangular portion is easy enough with just 2L.
So, the total circumference must be 100 and is:
Now, for the area:
The area of the entire region is
Since we must maximize area, solve  for L or W (whatever you want or whatever is easiest) and sub into the area equation. It will then be entirely in terms of one variable which you can differentiate and find the solution to the problem.