The ornamental design for a backpack, as shown and described.
FIG. 1 is a rear view of a backpack;
FIG. 2 is a left side view of the backpack of FIG. 1;
FIG. 3 is a front view of the backpack of FIG. 1;
FIG. 4 is a right side view of the backpack of FIG. 1;
FIG. 5 is a perspective top, rear, left view of the backpack of FIG. 1;
FIG. 6 is another front view of the backpack of FIG. 1;
FIG. 7 is a top view of the backpack of FIG. 1;
FIG. 8 is a bottom view of the backpack of FIG. 1; and,
FIG. 9 is a rear view of the backpack of FIG. 1, with a main compartment opened.
The dash-dot lines showing a human form in the figures are for the purposes of illustrating environment and form no part of the claim. The broken lines shown on the backpack are for the purpose of illustrating stitching and form no part of the claim.