Answer: 9 centimeters
Explanation:
Hi, to answer this question we have to apply the same ratio applied to find the height in centimeters of the door. 
12ft/36cm = 9ft/x cm 
12/36= 9/x 
x = 9/ (12/36) 
x =27cm 
Using the value of the window's wide instead of the door's height. 
12/36 = 3/x 
x = 3 / (12/36) 
x = 9cm 
Feel free to ask for more if needed or if you did not understand something.