The ornamental design for a back for a foldingchair, as shown and described.
FIG. 1 is a left side perspective view for a back for a folding chair showing our new design;
FIG. 2 is a front view of the back for a folding chair shown in FIG. 1;
FIG. 3 is a rear view of the back for a folding chair shown in FIG. 1;
FIG. 4 is a right side view of the back for a folding chair shown in FIG. 1;
FIG. 5 is a left side view of the back for a folding chair shown in FIG. 1;
FIG. 6 is a top view of the back for a folding chair shown in FIG. 1; and,
FIG. 7 is a bottom view of the back for a folding chair shown in FIG. 1.
The broken line illustrations in the drawing figures of the chair frame and seams in FIGS. 1-7 are for illustrative purposes only and form no part of the claimed design.