You would need a band pass filter followed by a detector circuit. You will also need to get a more precise specification of the requirement from your teacher. You need to know the approximate amplitude of the signal and also how close to 1.4 Mhz it needs to be. For example if it has to reject signals only a few hz away fro 1.4 Mhz then it will be hard to design. If detecting signals from 1.3 to 1.5 Mhz is good enough then it will be much easier.