I would like help to explain how to figure out the questions given.

For the following exercises, consider a wire long cut into two pieces. One piece forms a circle with radius and the other forms a square of side X You have 400 of fencing to construct a rectangular pen for cattle. What are the dimensions of the pen that maximize the area? How did you guys know what equations to use in these questions? I kind of understand the problem once the equations used to solve the problem are made but I have a problem actually finding the equations needed