input: Write a python function to perform addition and multiplication using a class.
  assert Calculate(3, 4).add() == 7
  assert Calculate(3, 4).multiply() == 12
  assert Calculate(5, 6).add() == 11
  assert Calculate(5, 6).multiply() == 30
output: class Calculate:
      def __init__(self, a, b):
          self.a = a
          self.b = b
  
      def add(self):
          return self.a + self.b
  
      def multiply(self):
          return self.a * self.b