I claim the ornamental design for a backpack, as shown and described.
FIG. 1 is a Perspective view of a backpack showing my new design;
FIG. 2 is a Front view of the backpack as shown in FIG. 1;
FIG. 3 is a Back view of the backpack as shown in FIG. 1;
FIG. 4 is a Top view of the backpack as shown in FIG. 1;
FIG. 5 is a Bottom view of the backpack as shown in FIG. 1;
FIG. 6 is a Right-side view of the backpack as shown in FIG. 1; and,
FIG. 7 is a Left-side view of the backpack as shown in FIG. 1.
The broken lines encircling unshaded portions of the backpack and the unshaded surfaces inside of these broken lines form no part of the claim. The broken lines shown on claimed surfaces depict stitching and form no part of the claim.