code for diagram

> AA:=[0,0]: BB:=[4,0]: CC:=[4,3]:
tri:=plots[polygonplot]([AA,BB,CC]):
txttxt:=plots[textplot]({[2,-.5,`x`],
[2,2,`y`],[4.2,1.5,`3`]},font=[TIMES,BOLD,16],
thickness=2):
arrw:=plottools[arrow]([4,3],[4.5,3],.03,.2,.1,
color=blue):
plots[display]({tri,txttxt,arrw},scaling=constrained,
axes=none);