The ornamental design for a lantern with extendablearm, as shown and described.
FIG. 1 is a top perspective view of the lantern with extendable arm showing our new design, with the extendable arm in a retracted position.
FIG. 2 is a bottom perspective view thereof.
FIG. 3 is a front side elevation view thereof.
FIG. 4 is a back side elevation view thereof.
FIG. 5 is a right side elevation view thereof.
FIG. 6 is a left side elevation view thereof.
FIG. 7 is a top plan view thereof.
FIG. 8 is a bottom plan view thereof; and,
FIG. 9 is right side elevation view thereof, with the extendable arm of the lantern in an extended position with the globe portion positioned at a top of the arm.
The broken lines in the figures show portions of the lantern with extendable arm and environment that form no part of the claimed design.