Answer:
12
Step-by-step 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
(x - 9)² + (y - 4)² = 36 ← is in standard form
with r² = 36 ⇒ r = = 6
diameter = 2 × r = 2 × 6 = 12