The ornamental design for a backpack, as shown and described.
FIG. 1 is a front top right side perspective view for a backpack, showing my new design;
FIG. 2 is a front top left side perspective view of FIG. 1 thereof;
FIG. 3 is a front perspective view of FIG. 1 thereof;
FIG. 4 is a rear perspective view of FIG. 1 thereof;
FIG. 5 is a left side perspective view of FIG. 1 thereof;
FIG. 6 is a right side perspective view of FIG. 1 thereof; and,
FIG. 7 is a top perspective view of FIG. 1 thereof.
The broken lines indicate unclaimed portions of the design and form no part of the claimed design.
Applicant respectfully submits the attached black and white photographs as formal figures for this case under 37 CFR §1.84(b)(1) and §1.152; , as these photographs are the only practicable medium for illustrating the external ornamental features and textures of the claimed invention.