The ornamental design for a backpack, as shown and described.
FIG. 1 is a rear plan view of a backpack showing my new ornamental design in a first state, comprising an insert portion that is configured to be received by a clear sleeve portion of a backpack included in the backpack;
FIG. 2 is a rear plan view thereof in a second state, comprising the insert portion having been received by the clear sleeve portion of the backpack;
FIG. 3 is a front plan view thereof in the first state or the second state;
FIG. 4 is a right-side elevation view thereof;
FIG. 5 is a left-side elevation view thereof;
FIG. 6 is a top elevation view thereof;
FIG. 7 is a bottom elevation view; and,
FIG. 8 is a perspective view of the backpack in the second state.
The process or period in which one state transitions to another state forms no part of the claimed design.
The broken lines on claimed surfaces, shown inside of solid lines, depict portions of the article and form no part of the claim. The surfaces inside of these broken lines form no part of the claim. The surfaces inside solid lines and adjacent broken lines are claimed.