The Pythagorean Theorem states that:
h^2=x^2+y^2
Which just means that the square of the hypotenuse, or longest side of a triangle is equal to the sum of the squares of the two other sides...
Often it is just called the "distance formula" because it is used so often in that context...
In this case we wish to know the distance between two points and the equation becomes...
d^2=(y2-y1)^2+(x2-x1)^2
d^2=(-6--6)^2+(-1-8)^2
d^2=(0)^2+(-9)^2
d^2=81, since d>0
d=+√81
d=9 units