How to shift signal to baseband using local oscillator frequency?
The simplest signal that has the spectrum shown is a 100 MHz carrier that is amplitude modulated with 100 kHz bandwidth information. If Fc = carrier frequency, a modulating frequency Fm causes sidebands at (Fc + Fm) and (Fc - Fm). To demodulate the signal, mix it with a local oscillator at Fc. You can write a formula for the output of the demodulator by representing each frequency as a trig function of time i.e. sine(2 pi F) or cosine(2 pi F). The important output is sine(2 pi Fm) which shows the modulating signal is recovered, which is what is meant by baseband. There are also higher frequency products but these will be filtered away in a receiver. See http://en.wikipedia.org/wiki/Sideband
or see http://en.wikipedia.org/wiki/Amplitude_modulation for a full math analysis that also considers phase.