input: Write a python function to convert currencies using an exchange rate.
  assert convert_currencies('USD', 'EUR', 0.9) == 0.9
  assert convert_currencies('GBP', 'USD', 1.3) == 1.3
  assert convert_currencies('JPY', 'AUD', 0.012) == 0.012
output: def convert_currencies(from_currency, to_currency, amount):
      return amount