Find the interior angles of the triangle with vertices (-4,1), (1,-6), and (5,-1)

Answer 1

If we label the points A,B,C, then
AB=√84
BC=√41
AC=√85

85 = 84+41-2√84√41 cosB

Now you can use law of sine or cosine for one other angle, and then the angles sum to 180.