Basic algebra.

Predicting the gain just by looking at the circuit is a skill that comes with an understanding of circuit theory and some practice. This particular problem is a good example of how it works. But first you have to have a firm grasp of the concept of impedance as a function of frequency. If you don't fully understand that, then you can't understand what all the parts in the circuit are doing. In that case, just stick with the the math and work it out.