Find the object distances (in terms of f) for a thin converging lens of focal length f

Question:

Find the object distances (in terms of f) for a thin converging lens of focal length f if
(a) The image is real and the image distance is four times the focal length;
(b) The image is virtual and the image distance is three times the focal length.
(c) Calculate the magnification of the lens for cases (a) and (b).