We claim the ornamental design for a washboard for washing machine, as shown and described.
FIG. 1 is a front perspective view of a washboard for washing machine showing our new design;
FIG. 2 is a front view thereof;
FIG. 3 is a rear view thereof;
FIG. 4 is a left-side view thereof;
FIG. 5 is a right-side view thereof;
FIG. 6 is a top view thereof;
FIG. 7 is a bottom view thereof; and,
FIG. 8 is a front perspective view of the washboard for washing machine in which a washing machine is provided in broken lines to illustrate an example environment and form no part of the claimed design.
The broken lines in the figures show portions of the washboard for washing machine which form no part of the claimed design.