Complete Rectangle’s two member functions: area (width times height) and perimeter (twice the sum of width
and height).
This is deliberately similar to the last exercise, with two member functions on one class instead of one – both
read the same two member variables, computing something different from them. Notice main never touches width
or height inside a formula itself; it just asks the object for area() and perimeter() and trusts the class to
know how those are computed. That’s the actual value of bundling data and behavior together, made concrete.