Backpack

FIG. 1 is a first perspective view of a backpack showing my new design; FIG. 2 is a second perspective view thereof; FIG. 3 is a front view thereof; FIG. 4 is a back view thereof; FIG. 5 is a right side view thereof; FIG. 6 is a left side view thereof; FIG. 7 is a top view thereof; and, FIG. 8 is a bottom view thereof. The broken lines shown in the drawings depict portions of the backpack that form no part of the claimed design.