Question 135147
Let's name the points
A (13,-1)
B (-9,3)
C (-3,-9)
For a right triangle the sums of the squares of the two legs equal the square of the hypotenuse, the longest leg.
Calculate each leg length first using the distance formula between two points.
{{{D^2=(x[1]-x[2])^2+(y[1]-y[2])^2}}}
AB, the distance from point A to point B. 
{{{(AB)^2=(13-(-9))^2+(-1-3)^2}}}
{{{(AB)^2=(22)^2+(4)^2}}}
{{{(AB)^2=484+16}}}
{{{(AB)^2=500}}}
AC, the distance from point A to point C. 
{{{(AC)^2=(13-(-3))^2+(-1-(-9))^2}}}
{{{(AC)^2=(16)^2+(8)^2}}}
{{{(AC)^2=256+64}}}
{{{(AC)^2=320}}}
BC, the distance from point B to point C. 
{{{(BC)^2=(-9-(-3))^2+(3-(-9))^2}}}
{{{(BC)^2=(-6)^2+(12)^2}}}
{{{(BC)^2=36+144}}}
{{{(BC)^2=180}}}
Since AB is the largest, it is the hypotenuse. 
If it is a right triangle then,
{{{(AB)^2=(AC)^2+(BC)^2}}}
{{{500=320+180}}}
{{{500=500}}}
True statement. 
Those point, therefore, define a right triangle.