1. Someone has written 7.3 m at the slanted edge of the prism. That's definitely wrong.
2. To calculate the length of the slanted edge you have to use Pythagorean theorem. I've indicated the right triangle in question ion green. You are supposed to know why the short leg is 0.55 m. The length of the hypotenuse (that's the slanted edge) is approximately 7.2209 m (the exact value - if the given lengthes are exact! - is )