The answer is: 12 centimeters.
 The explanation is shown below:
 1. By definition, all four sides of the square are equal and the diagonals are equal too. 
 2. Keeping the information above on mind, you know that AC and BD are equal:
 

 3. Then, the length of BD is twice BE. So, you can calculate it as following:
 

 

 4. Therefore, the lenght of AC is:
 
