The ornamental design for back plate for a hook to hang on a wall, as shown and described.
FIG. 1 is a front view of back plate for a hook to hang on a wall embodying my new design;
FIG. 2 is a left side view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a plan view thereof;
FIG. 5 is a bottom view thereof;
FIG. 6 is an environmental, perspective view thereof; and,
FIG. 7 is a reduced scale environmental, perspective view.
The broken line showing of partial structure and a hook is included for the purpose of illustrating environment and forms no part of the claimed design.