Answer:
radius = 6
Explanation:
The equation of a circle in standard form is
(x - h)² + (y - k)² = r²
where (h, k) are the coordinates of the centre and r is the radius
Given
x² - 12x + y² + 4y = - 4
Using the method of completing the square on the x and y terms
add (half the coefficient of the x/y term )² to both sides
x² + 2(- 6)x + 36 + y² + 2(2)y + 4 = - 4 + 36 + 4, that is
(x - 6)² + (y + 2)² = 36 ← in standard form
with r² = 36 ⇒ r =
= 6