Because there's no triangle command in freebasic.
http://www.freebasic.net/wiki/wikka.php?wakka=DocTocThere are several triangle routines posted on this forum, if you spec one up I'll write one for you if not.
Are you just using fbgfx?
Also, FB is a bit different, say you have
rect 10,10 to 100,100
in yabasic, that would likely look like
rect(10,10,100,100)
in FB. There's no rect command either.
That was the great advantage of PS2 yabasic - having all those built in graphics commands.
Jim