Sometimes it is necessary to raise an exception. When you do this, you should always include a meaningful error message to indicate what the source of the error is. This makes your code more readable and helps significantly with debugging. For situations where you know that the error source will be a certain type, you can choose to raise one of the built in error types, but should still include a meaningful message.
This particular exercise requires that you use the raise statement to "throw" a ValueError if the question passed to answer() is malformed/invalid, or contains an unknown operation. The tests will only pass if you both raise the exception and include a message with it.
To raise a ValueError with a message, write the message as an argument to the exception type:
# if the question contains an unknown operation.
raise ValueError("unknown operation")
# if the question is malformed or invalid.
raise ValueError("syntax error")