The ornamental design for protective cover for a tabletcomputer, as shown and described.
FIG. 1 is a perspective view of the protective cover for a tablet computer showing our new design;
FIG. 2 is a top plan view thereof;
FIG. 3 is a bottom plan view thereof;
FIG. 4 is a front elevational view thereof;
FIG. 5 is a rear elevational view thereof;
FIG. 6 is a left side elevational view thereof; and,
FIG. 7 is a right side elevational view thereof.
The broken lines in the drawings depict environmental subject matter/or parts of the protective cover for a tablet computer only and form no part of the claimed design.