input: Write a python function to calculate the area of a rectangle using a class.
  assert Rectangle(5, 6).calculate_area() == 30
  assert Rectangle(7, 3).calculate_area() == 21
  assert Rectangle(10, 2).calculate_area() == 20
output: class Rectangle:
      def __init__(self, width, height):
          self.width = width
          self.height = height
  
      def calculate_area(self):
          return self.width * self.height