50g

I think that this little program can help you... it`s from a
minichallenge realized in a latin american web page...

As arguments you have to put the three points in lists in the
following way:

7:
6:
5:
4:
3:         {x1 y1}
2:         {x2 y2}
1:         {x3 y3}

It`s the source code...

\<< + + EVAL SWAP DUP
SQ 1 \->V3 4 ROLL DUP
SQ 1 \->V3 6 ROLL DUP
SQ 1 \->V3 SWAP ROT 3
ROW\-> INV 4 ROLLD
\->V3 * -3 FS?C 'Y'
ROT V\-> SWAP 'X^2' * 
ROT 'X' * + SWAP + 
= SWAP
  \<< -3 SF
  \>> IFT
\>>

Greetings from Venezuela...