The ornamental design for a garment with back opening, as shown and described.
FIG. 1 is a front, perspective view of a garment with back opening showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a right side view thereof;
FIG. 5 is a left side view thereof;
FIG. 6 is a top view thereof; and,
FIG. 7 is a bottom view thereof.
In the drawings, the broken lines depict unclaimed subject matter only and form no part of the claimed design. The long dash-short dash bolded broken lines indicate the bounds of the claimed design and otherwise form no part of the claimed design.