I did stuff, and here's what I got:
First of all, apparently you can't even make a trapezoid with your measurements; only two sides are congruent, and none can be parallel. Secondly, you can draw a figure like this using the 3-4-5 right triangle:
C
4 /
/ D
|\
4 | \ 5
|__\
A 3 B
Pretend there is a line connecting B and C that measures 6
Now we know the length of DB, 5. This means that DB cannot be the longest diagonal. The longest diagonal has to be longer than any of the sides. So we have to find a number that is greater than 6 BUT less than 8, because of the triangle inequality theorem (thanks) which states that the sum of any 2 sides must be greater than the 3rd side. The sum of the sides of the triangle formed is 4+4, or 8, which means it can't be greater than 8. And since its length is an integer, AC has to be 7. That's all I could think of.