If the radius of the sphere S is 8 units, what is the value of k?

For sphere x² + y² + z² − 4x − 6y − 12z + k = 0, center is (2, 3, 6) and radius = √(4 + 9 + 36 − k) = √(49 − k). Given radius = 8, so 49 − k = 64, giving k = −15.