In a rectangle, the diagonals are equal. You need to set the two equal to each other. The line BE would be doubled to equal AC. Then solve:
2(2x-3)=3x+1
4x-6=3x+1
x=7
Now that you have x, you can find AC. Just plug it in.
3(7)+1
21+1
22
There you have it! AC=22