Diameter= 18cm
Radius is half of diameter
Radius=9cm
Answer:

Step-by-step explanation:
Given
Empty Tank

When filled with water, the height is:

Required
Determine how many more cubic inches needed
First, we calculate the difference between the height of the tank and the height of the water.


The volume of water to full the tank is then calculated as:



The graph suggests that the two lines meet at 
If this is true, that point must belong to both lines.
To check this, plug
in both equations, and you must get
once you simplifiy all the numbers.
In the first equation we have

In the second,
