Answer: 8
The point E in the parallelogram is located at the crossing point of the diagonal line. It can be concluded that the distance of AE is equal to CE, or equal to 1/2 AC. Then you can get three equation like this:
AE=CE
AE=x^2−8
CE=2x .
Using these equation you can find the value of x
AE= CE
x^2−8 = 2x
x^2−2x-8 = 0
(x- 4) (x+2)
x1= 4 x2= -2
Excluding the minus result, you can get that AE= CE= 4
Then AC would be:
AC= AE+CE
AC= 4 + 4= 8
