You will need
The first method. The focal length can be found using the thin lens formula: 1/distance from lens to object+1/distance from lens to image=1/focal length of the lens. From this formula we Express the focal length of the lens. You should now have the following formula: focal length of the lens=distance from lens to image*distance from lens to object/( distance from lens to image+the distance from the lens to the subject). Now count the unknown value using the calculator.
If you do not thin, but a thick lens, then the formula remains unchanged, but the distances are measured not from the center of the lens and principal planes. For real image from the actual subject in the gathering lens focal length take as a positive value. If the scattering lens - the focal length is negative.
The second method. The focal length can be found using the formula of image scale: scale=focal length of lens/distance from the lens to the image-focal length lenses) or scale=(distance from lens to image-focal length of lens)/focal length of the lens. Expressing this formula focal length, you can easily count it.
The third way. The focal length can be found using the formula optical power of a lens: the optical power of the lens=1/focal length. Express this formula focal length: focal length=1/focal power. Count.
The fourth method. If you are given the thickness of the lens and the magnification, to find the focal length, multiply them.
Now you know how to find the focal length. Choose one or the other of the above method depending on what is given to you, and then you can easily choose the task set before you. Be sure to identify what is in front of you lens, as it depends on a positive or negative value has a focal length. And then you decide it without a single mistakes.