Say we have an object $O$ that is to the left of two lenses. Let's say that the first lens $L_1$ is to the left of $L_2$, the second lens. Assuming we know the type of each lens (concave or convex), the focal length of each lens and the distances between the object and the lenses, we could determine the final image as follows: First, use the lens equation to find the image of the object through $L_1$, and call this image $I_1$. Then you can treat $I_1$ as the object of the second lens $L_2$ by using the distance between $I_1$ and $L_2$. Doing so will produce your final image $I_2$.
What if I decided to go through the same process, but by finding the image that goes through $L_2$ first and then using that image as the object for $L_1$? Does this calculation correspond to a valid image? If not, then why?
Also, I ask the same questions for the case when $L1$ is a lens and $L2$ is a mirror.
I am sorry if this is somewhat basic, but it is bothering me. Thank you for your help in advance!