Hello,
is that an exam question?
Think about it:An X-Ray of what? The arm? Just kidding. It depends on the X-Ray. If you get an X-Ray of the whole right leg and an X-Ray of the whole left leg, then you can take a tape measure and measure the length of the bones. If there is a difference, you can determine that there is a leg length discrepancy ... this is structural.if an X-ray alone can show a leg length difference
Now you can have a think and come up with the explanation what a functional leg difference is.