The ornamental design for the back and seat for a chair, as shown and described.
FIG. 1 is a perspective view showing the right side, top and front of the back and seat for a chair which forms my new design, the long broken lines in this and other figures indicating unclaimed environment and forming no part of the claim, the short broken lines in this and other figures is understood to represent stitching, and the cross hatching in this and other figures is understood to represent mesh;
FIG. 2 is a perspective view showing the left side, top and rear thereof;