Anyone know a good way to find the focal length of a lens?

I have a 3mm light source
It's 3cm - 9cm away from the unknown lens (this is set and does not adjust. Typically 6cm away)
Unknown lens diameter is 28mm
It's 141 cm to 226cm away from the sensor.
A solid 28mm - 30mm light will be shown.

It's been killing me for years.