Calculating the frequency of a sinewave with the cutoff frequency, the input and output amplitudes of the sinewave?
Find the attenuation factor for your filter at the given frequency, which is usually expressed in dB. Then use this equation:

\$ dB = 20 \log\frac{V_{out}}{V_{in}}\$

If you need to know what the dB attenuation factor is for your filter, use this guide

To find the frequency of a low pass filter, you use this equation

\$  \frac{V_{out}}{V_{in}} = \frac{1}{2\pi R C}\$