use midpoint equation
radius = distance between midpoint and one of the endpoints.
midpoint: (3+5)/2, (2+6)/2, (5+7)/2 = (4,4,6)
equation of sphere: (x-4)^2 + (y-4)^2 + (z-6)^2 = r^2
square distance between midpoint and one of the endpoints.
r^2 = 6
Equation: (x-4)^2 + (y-4)^2 + (z-6)^2 = 6
