Step-by-step explanation:
In order to find whether it will sink or float in water we must first find the density of the object
The density of a substance can be found by using the formula

From the question
mass = 18 g
volume = 9 cm³
So we have

We have the density as 2.0 g/cm³
The object will sink in water since it's density is greater than that of water which is 1.0 g/cm³
Hope this helps you