When the truck's weight is added to the boat, the boat sinks 5 cm deeper, and displaces additional water whose weight is equal to the weight of the truck.
The volume of the additional displaced water is
(3.9 m) x (6.3 m) x (5.0 cm)
= (3.9 m) x (6.3 m) x (0.05 m) = 1.2285 m³ .
The weight of that much water is the weight of the truck.