The ornamental design for a backpack, as shown and described.
FIG. 1 is a front perspective view of a backpack showing my new design;
FIG. 2 is a front view of the backpack of FIG. 1;
FIG. 3 is a rear view of the backpack of FIG. 1;
FIG. 4 is a left side view of the backpack of FIG. 1;
FIG. 5 is a right side view of the backpack of FIG. 1;
FIG. 6 is a top view of the backpack of FIG. 1;
FIG. 7 is a bottom view of the backpack of FIG. 1; and,
FIG. 8 is a front perspective view of the backpack of FIG. 1 showing the lid in the open position.
The dash-dash broken lines depict stitching of the backpack that form no part of the claimed design. The dot-dot-dash broken lines depict portions of the backpack that form no part of the claimed design.