An Athletic Field Is To Be Built In The Shape Of A Rectangle Area Of An Athletic Field?

Area of an Athletic Field? - an athletic field is to be built in the shape of a rectangle

A sports field is in the shape of a rectangle length x units to be built by the semi-circle of radius r bounded at both ends. The field must be returned by the circuit 400-M.

a) Express the area of the rectangular box with the help of x alone or lonely.

b) What are the values of x and R are the rectangular part of the largest area possible.

^ I do not understand how to use (Part B, optimize) to.