Final answer:
To find the density of a seashell, Alisa needs a graduated cylinder to measure its volume using the water displacement method, alongside the balance she has for measuring mass.
Step-by-step explanation:
To determine the density of the seashell, Alisa needs not only a balance to measure its mass but also a method to determine its volume. One common approach to measuring the volume of an irregularly shaped object like a seashell is the water displacement method. For this, Alisa would need a graduated cylinder filled with a known volume of water. She would then submerge the seashell in the water, and the change in water level would indicate the volume of the seashell. The density can then be calculated using the formula density = mass/volume.