using convex lens find the focal length of a concave lens

If the two lenses form an image when they are combined close together (ie the combined lens is convergent) you can measure the focal length of the combination and use the relationship 1/fc = 1/f1 + 1/f2 where fc is the combined focal length and f1 and f2 are the focal lengths of the two elements. This works for thin lenses close together.

You can also do it by using an image formed by the convergent lens as the object for the divergent lens, then applying the thin lens equation after measuring the location of the new image. The divergent lens is placed between the convergent lens and the image it forms.