Answer: 12 cm
Step-by-step explanation: In order to do this, you have to know that the formula for the area of a circle is pir^2. Pi times the radius squared is equal to 36 pi. Well, 6 squared is 36, and 36 times pi is 36 pi. So that means that the radius is 6. Then, the radius is half the diameter, so the diameter is 12.