I claim the ornamental design for the backpack, as shown and described.
FIG. 1 is a top, front, right perspective view of a backpack of my new design;
FIG. 2 is a bottom, rear, left side perspective view thereof;
FIG. 3 is a front view thereof;
FIG. 4 is a rear view thereof;
FIG. 5 is a left view thereof;
FIG. 6 is a right view thereof;
FIG. 7 is a top view thereof; and,
FIG. 8 is a bottom view thereof.
The broken lines in the figures depict portions of the backpack that form no part of the claimed design. The zipper stitching shown in the figures also forms no part of the claimed design.