A mid-cross-section of the can with the balls in it

would look like this



So we can see that the can is 18cm wide, which means its diameter
is 18cm.

And if we draw the green lines vertically:



We see that the can is 2×18 or 36cm high.

The volume of a cylinder is



Since the diameter of the can = 18cm, the radius of the can = 9cm.

So we substitute r = 9 and h = 36, and 



to the nearest cubic centimeter.

Edwin