We claim the ornamental design for a back assembly, as shown and described.
FIG. 1 is a perspective view of a back assembly on a stool showing a top, a back and a left side.
FIG. 2 is another perspective view of the back assembly of FIG. 1 showing a front, the top and the left side.
FIG. 3 is left side elevational view, the right side being a mirror image.
FIG. 4 is a rear elevational view.
FIG. 5 is a top plan view
FIG. 6 is a front elevational view; and,
FIG. 7 is a bottom plan view.
The broken line showings of an actuator and a mounting portion form no part of the claimed design. The broken line showing of a portion of the stool illustrates the environment of the claimed design and forms no part thereof.