The ornamental design for a backpack, as shown and described.
FIG. 1 is a top, front, right perspective view of a backpack showing our new design;
FIG. 2 is a top front, left perspective view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is top, front, left perspective view of the backpack shown in an alternate configuration.
The uneven-length broken lines immediately adjacent to the shaded area represent unclaimed boundaries of the claimed design. The even-length broken lines showing the Remainder of the backpack are for environmental purposes only and form no part of the claimed design.