The ornamental design for an exerciseplatform, as shown and described.
FIG. 1 is a perspective view of an exercise platform, showing our new design;
FIG. 2 is a top view thereof;
FIG. 3 is a bottom view thereof;
FIG. 4 is a left side view thereof;
FIG. 5 is a right view thereof;
FIG. 6 is a front view thereof; and,
FIG. 7 is a back view thereof.
The heavy dotted line portion shown is surface pattern that forms part of the claimed design. The lightweight dashed broken line portion of the figure drawings is included to show environment that forms no part of the claimed design.