input: Write a python function to perform addition and subtraction based on an operation string.
  assert math_operation(5, 3, 'add') == 8
  assert math_operation(5, 3, 'subtract') == 2
  assert math_operation(10, 5, 'add') == 15
  assert math_operation(10, 5, 'subtract') == 5
output: def math_operation(a, b, operation):
      if operation == 'add':
          return a + b
      elif operation == 'subtract':
          return a - b
      else:
          raise ValueError("Invalid operation")