input: Write a python function to find the greatest common multiple (GCM) of two integers using the greatest common divisor (GCD).
  assert greatestCommonMultiple(8, 12) == 24
  assert greatestCommonMultiple(21, 6) == 42
  assert greatestCommonMultiple(9, 28) == 252
output: def greatestCommonMultiple(a, b):
      def gcd(x, y):
          while y:
              x, y = y, x % y
          return x
      
      return abs(a * b) // gcd(a, b)