the ornamental design for back for an armchair, as shown and described.
FIG. 1 is a perspective view showing the new design;
FIG. 2 is a front view thereof it being understood that the rear is a
mirror image of that shown.
FIG. 3 is a top view thereof; and,
FIG. 4 is a side view thereof, the opposite side view being a mirror image
thereof.
The broken line showing of the environmental structure of FIG. 1 of the
drawing for illustrative purposes only and forms no part of the claimed
design.