The ornamental design for a tabletholder for backpack, as shown and described.
FIG. 1 is a front view of a tablet holder for a backpack showing the new design;
FIG. 2 is a front view thereof;
FIG. 3 is a front perspective view thereof, with an opening for accepting a tablet;
FIG. 4 is a front perspective view thereof, with an opening for accepting a tablet; and,
FIG. 5 is a side view thereof.
Aspects shown in broken lines, the electronic tablet device and an image displayed on the tablet device screen in FIG. 2, are intended to depict environment and are for illustrative purposes only and form no part of the claimed design.