The ornamental design for a supportingleg for a serving tray, as shown and described.
FIG. 1 is an end elevational view of a supporting leg for a serving tray,
showing my new design, the opposite end being a mirror image thereof;
FIG. 2 is a left side elevational view;
FIG. 3 is a right side elevational view; and,
FIG. 4 is a bottom perspective view thereof.
The top and bottom of the support leg are plain and unadorned. The broken
lines in FIG. 1 are for environmental purposes only and form no part of
the claimed design.