An object is placed in front of a converging lens in such a position that the lens (f = 15.0 cm) creates a real image located 21.0 cm from the lens. Then, with the object remaining in place, the lens is replaced with another converging lens (f = 16.0 cm). A new, real image is formed. What is the image distance of this new image?
this is familiar with what i've been working with, but this time there is only one answer and two focal points, so no need to use one focal point, but both! I was unsure how to use both focal points for one answer.