Changing a line with cumtrapz

If I have a line on a graph (below) where

• L=5,
• M2=-(w*L^2)/8
• w=5

line([0 L/2],[0 M2],'Color','b','linewidth',1.5);

How do I implement the Cumtrapz function to make this line a parabola to start at x=0 coordinate, y=0 coordinate and end at x=L/2 coordinate, y=M2 coordinate?