The ornamental design for a backpack, as shown and described.
The application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.
FIG. 1 is a front view of a backpack showing our new design;
FIG. 2 is a rear view thereof showing the shoulder straps in a second position;
FIG. 3 is a top view thereof;
FIG. 4 is a bottom plan view thereof in an alternate position with the shoulder straps unfastened;
FIG. 5 is a left side view thereof with the shoulder straps in the position of FIG. 2;