The ornamental design for a ridinglawnmower, as shown and described.
FIG. 1 is a right side, elevational view of a riding lawn mower showing our new design;
FIG. 2 is a left side, elevational view thereof;
FIG. 3 is a back side, elevational view thereof;
FIG. 4 is a front side, elevational view thereof;
FIG. 5 is a top plan view thereof; and,
FIG. 6 is a perspective view thereof.
Any elements shown in dotted and/or dashed lines are provided for illustrative purposes only and form no part of the design claimed. Any surface shading is used solely to show the character or contour of the surfaces represented.