FIG. 1 is a perspective view of a supporting bracket showing my design in one state. FIG. 2 is a front elevational view thereof. FIG. 3 is a rear elevational view thereof. FIG. 4 is a left-side elevational view thereof. FIG. 5 is a right-side elevational view thereof. FIG. 6 is a top plan view thereof. FIG. 7 is a bottom plan view thereof; and, FIG. 8 is a perspective view thereof shown in another state.