Passive Pi Low Pass Filter LC
The impedance magnitude of a capacitor is:

&nbsp; &nbsp; ZC = 1 / &omega;C

The impedance magnitude of a inductor is:

&nbsp; &nbsp; ZL = &omega;L

The rolloff frequency of a LC filter is when these two are equal.  Set the two equations above equal to each other, and solve for &omega;.  Then remember that &omega; = 2&Pi;f, where f is the frequency in Hz.