The ornamental design for a writingtabletsupport, as shown and described.
FIG. 1 is a perspective view of a writing tablet support showing my new design;
FIG. 2 is a bottom plan view thereof;
FIG. 3 is a rear elevational view thereof;
FIG. 4 is a front elevational view thereof;
FIG. 5 is a right-side elevational view thereof;
FIG. 6 is a left-side elevational view thereof; and,
FIG. 7 is a perspective view of a second embodiment of FIG. 1, the configuration of the alignment to the other edge of the supported tablet, it being understood that all other surfaces are the same as those of the first embodiment.
The broken lines are included to show environment and form no part of the claimed design.