The ornamental design for a extendabletable, as shown and described.
FIG. 1 is a front perspective view of an extendable table showing our new design, a rear perspective view being a mirror image thereof; and,
FIG. 2 is an enlarged partial view showing the basket weave-like edging of the table top which is continuous along the front, rear and both side edges of the table top of the table of FIG. 1.