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:
[tex]AC=BD[/tex]
3. Then, the length of BD is twice BE. So, you can calculate it as following:
[tex]BD=2BE[/tex]
[tex]BD=2(6cm)\\BD=12cm[/tex]
4. Therefore, the lenght of AC is:
[tex]AC=12cm[/tex]
