We can use the pythagorean theorm. This means that the points can form the two legs of a triangle.
a = 6
b = 6
c = length AB
a^2+b^2 = c^2
2(6^2) = c^2
72 = c^2
c would be
or 
a = 6
b = 6
c = length AB
a^2+b^2 = c^2
2(6^2) = c^2
72 = c^2
c would be