The ornamental design for a back and seat for a chair, as shown and described.
FIG. 1 is a top, right, front perspective view of a back and seat for a chair showing my new design;
FIG. 2 is a top, left, rear perspective view 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 long broken lines in the figures represent environemt that forms no part of the claimed design. The short broken lines in the figures represent stitching.