Final answer:
Matri's rectangular boat sank because its density with the added block was too high, resulting in insufficient water displacement for buoyancy. To fix this, she could adjust the boat's weight or volume to enhance the buoyant force and enable it to float.
Step-by-step explanation:
Matri's boat began to sink because the combined density of the boat and the block was greater than the density of the water, which resulted in the boat displacing not enough water to support its weight, according to Archimedes' Principle. To remedy this, Matri could distribute the weight more evenly, reduce the weight of the boat or the block, or redesign the boat with a greater volume to displace more water and increase the buoyant force. Creating a shape that displaces more water without increasing the mass too much will help her boat achieve buoyancy and float, much like how a clay boat or a steel ship can float despite their heavy materials.