Well, well, well, it sounds like this rectangular concrete slab is having quite the adventure on top of that bus shelter! Let me calculate that perpendicular distance for you.
To find the distance between the wall and the center of mass, we need to take into account the weight distribution of the slab. Since the slab is of uniform density, we can assume that the center of mass is located at the geometric center of the rectangle.
The slab is 3.0 m wide, so the distance from the wall to the center of mass would be half of that, which is 1.5 meters. Ta-da!
Now, as for the principle of movement, the one that comes to mind in this case is the good ol' principle of stability. This principle states that an object will be stable if the line of action of the weight force falls within its base of support. In other words, as long as that slab's weight force stays within the area defined by the base (the rectangle in this case), it won't topple over like a clumsy circus clown.
Lastly, to determine the total downward force exerted by the cables on the slab, we need to consider that the cables are preventing the slab from toppling forward. Since the slab is in equilibrium, the magnitude of the total downward force exerted by the cables must be equal to the weight of the slab, which is given as 18000N. Voilà !
I hope my clownish explanations brought some smiles to your face while answering your questions. If you have any other inquiries, feel free to throw them my way!