The ornamental design for a backpack, as shown and described.
FIG. 1 is a perspective view taken from the front, left side, and top side of the backpack of the present design, shown with its top opening in a closed position, in accordance with the present design.
FIG. 2 is another perspective view thereof, showing the backpack in an open configuration.
FIG. 3 is a front elevation view of the backpack of FIG. 1.
FIG. 4 is a rear elevation view of the backpack of FIG. 1.
FIG. 5 is a left side elevation view of the backpack of FIG. 1.
FIG. 6 is a right side elevation view of the backpack of FIG. 1.
FIG. 7 is a top plan view of the backpack of FIG. 1; and,
FIG. 8 is a bottom plan view of the backpack of FIG. 1.
The dash-dash broken lines shown represent stitching and form part of the claimed design. The dot-dash lines show portions of the backpack that form no part of the claimed design.