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:

The answer to this equation would be -8/35
Here, f(x) = kx
k = f(x)/x
k = 48/8
k = 6
So, when x = 2,
f(x) = 6 * 2
z = 12
In short, Your Answer would be: 12
Hope this helps!