Final answer:
The question does not provide enough information to solve for the focal length of the mirror. If the mirror is a plane mirror, the focal length concept is not applicable, and the image will be upright and the same size as the ball. For a spherical mirror, more information on object/image distances would be needed.
Step-by-step explanation:
To find the focal length of the mirror when the ball's own image is formed at the wall, we need further clarification about the type of mirror used in the scenario. However, assuming a spherical mirror and using information similar to the problem stated, we can infer that if an image is formed at the wall where the actual object is present, the mirror could be a plane mirror, in which case the focal length is not applicable. Plane mirrors produce images at the same distance behind the mirror as the object is in front of it, and the size and nature of the image are the same as the object (upright and same size). If it were a converging mirror, we would expect the object distance (o), image distance (i), and focal length (f) to be related by the mirror equation 1/f = 1/o + 1/i, but without additional information about the distances, we cannot solve for the focal length.