The ornamental design for an extendabletable, as shown and described.
FIG. 1 is a front perspective view of an extendable table with top plates seated on the table top; and,
FIG. 2 is a partial enlarged front perspective view of the table of FIG. 1 showing one corner of the table of FIG. 1 showing the pattern of the edge molding of the table top.
The broken lines showing are for environmental purposes and form no part of the claimed design.