It's hard to believe you are serious- you can't draw? Draw a long line segment representing the mirror. Mark a point representing the camera and draw a perpendicular from it to the line segment. Label that "8.0 m" Draw another mark representing the actor and draw a perpendicular from it to the mirror. Label that "8.9 m". (Was that a typo? You say "also". Did you mean to say "also 8.0 m"?). Draw a line from the camera to the actor and label it "12.0 m". Finally, draw the "light ray" from the actor to the mirror to the camera. That is, one line from the actor to the mirror and another from that point on the mirror to the camera. Remember that the angles the two lines make with the mirror must be equal. The total length of those two lines is the distance the camera must be focused for.
The general problem, which you would have if the two perpendicular distance were really 8.0 m and 8.9 m is an exercise in trigonometry. If the two distances are really equal, both 8.0 m, then it is a relatively simple problem in symmetry.