Ah yes, good ol’ Pythagorean’s theorem.
The hypotenuse is the opposite of the 90 degree angle, the longest side of the triangle.
The legs are opposite of the acute angles
Formula: a^2 + b^2 = c^2
2:
Let’s plug in some values
(6)^2 + (3)^2 = c^2
C = 6.7
The side will be 6.7 units
3)
Let’s plug in some values
(13.3)^2 = (9.7)^2 + (b)^2
(13.3)^2 - (9.7)^2 = b^2
176.89 - 94.09 = b^2
b = 9.1
The side will be 9.1 units
Answer:

Step-by-step explanation:
The information that we kow about the shape of the aquarium:
length:

width:

when the reef is submerged, the water level rises 3 feet. The volume that those 3 ft represent in the aquarium will be the volume of the reef.
Thus, we calculate the volume of water "addition" with the same length and width as the aquarium but with a deep of 3 feet (thi is because the reef caused the water to rise 3 feet), and this is the volume of the reef that was added to the aquarium:

The volume of the stone is 