show that the following points are collinear are: (3,7),(6,5)and(15,-1)

=?(x2-x1)?+(y2-y1)?
=?(6-3)?+(5-7)?
=?(3)?+(2)?
=?9+4
=?13 units

=?(x2-x1)?+(y2-y1)?
=?(15-6)?+(-1-5)?
=?(9)?+(1+5)?
=?81+(6)?
=?81+36
=?117 units

=?(x2-x1)?+(y2-y1)?
=?(3-15)?+(7-(-1))?
=?(12)?+(7+1)?
=?144+(8)?
=?144+64
=?208 units