The graph of the equation below is a circle. what is the length of the radius of the circle? (x - 4)2 + (y + 12)2 = 172

Hello : 
an equation of the circle Center at the A(a,b) and ridus : r is :
(x-a)² +(y-b)² = r²
in this exercice : a =4   and b = -12 
 the length of the radius is : r = √172