Calculate Gain of Modified Band-Pass Filter
You need the complex frequency response, which you can get by using the voltage divider rule with complex impedances. You probably know that for a capacitor you have \$Z=1/(j\omega C)\$. This way you can express the ratio \$V_{out}/V_{in}\$ as a complex function of the independent variable \$\omega\$ (this is the complex frequency response). The gain for a given frequency is then the magnitude of the frequency response evaluated at that frequency.

If you do the math you should get for \$\omega=1/RC\$ a gain

$$G=\frac{A}{1+2A}$$