The ornamental design for a rearbumper for a motor vehicle, as shown and described.
FIG. 1 is a front perspective view for a rear bumper for a motor vehicle showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
The de-emphasized and halftone portions shown in the photographs illustrate environmental structure of the rear bumper for a motor vehicle that forms no part of the claimed design.