Here are the 4 inequality signs.

---> a is less than b

---> a is less than or equal to b

---> a is greater than b

---> a is greater than or equal to b
For the second and fourth lines above, the two sides may or may not be equal.
For the first and third lines above, the two sides cannot be equal.
648 cm
To find the volume of a rectangular prism (such as the one in the picture) you would do:
V=L*W*H
So plugging the numbers in:
V=18*12*3