Complete whatever squares you can:
x² - 12x = x² - 12x + 36 - 36 = (x - 6)² - 36
y² - 2y = y² - 2y + 1 - 1 = (y - 1)² - 1
So rewrite the equation as
x² + y² + z² = 12x + 2y
→ x² - 12x + y² - 2y + z² = 0
→ (x - 6)² - 36 + (y - 1)² - 1 + z² = 0
→ (x - 6)² + (y - 1)² + z² = 37
Then the center is (x, y, z) = (6, 1, 0), and its radius is √(37).
