The ornamental design for a back and seat for a chair, as shown and described.
FIG. 1 is a perspective view showing the right side, top and front of a back and seat for a chair which forms my new design;
FIG. 2 is a perspective view showing the left side, top and rear thereof;
FIG. 3 is a front elevational view thereof;
FIG. 4 is a right side elevational view thereof;
FIG. 5 is a rear elevational view thereof;
FIG. 6 is a left side elevational view thereof;
FIG. 7 is a top plan view thereof; and,
FIG. 8 is a bottom plan view thereof.
The longer broken lines in the figures depict the portions of the back and seat for a chair that form no part of the claimed design. The shorter broken lines in the figures represent stitching.