The error was the Jacob multiplied 2.5 by 3 instead of dividing. Instead it should be.
The mapping rule for a 180 degree rotation is:
(x,y) (-x,-y)
1. Multiply the x-coordinate by -1.
2. Multiply the y-coordinate by -1.
3. SImplify
So the coordinate after a 180 degree rotation is: (5,-1)
Rate of change is 4. change of y over change of x= 20/5
The third side of a triangle must always be less than the sum of the other 2 sides
here 8 + 8 = 16 and 17 ( the third side ) 17 > 16
Hence this is not a valid triangle