code for figure

> tri:=plots[polygonplot]([[0,0],[4,0],[4,3],[0,0]],scaling=constrained, axes=none,thickness=3 ):

> tritxt1:=plots[textplot]({[0,-.2,`(x1,x2)`], [2,-.2, `delta x`], [4.5,1.5,`delta y`,align=RIGHT],[4,3.2, `(x2,y2)`]}):

> plots[display]({tri, tritxt1});