The mattress of a water bed is 2m long by 2m wide and 30cm deep.

physics.nist.gov/cuu/Units/prefixes.html

You just have to memorize this stuff, at least the ones you work with all the time. It's ok to bookmark the page so you can look up the others when you run across them.

There is a prefix every three powers of ten, except for 100, 10, 1/10, and 1/100. To go from one to another you use the LARD rule: LEFT ADD RIGHT DEDUCT. When moving the decimal to the LEFT you ADD to the power of ten. When going to the RIGHT you DEDUCT from the power of ten.

30 centimeters = 0.3 meter

By definition a cubic meter of water masses 10^3 kilograms, so all you need to do is find the volume and convert that to weight.

2 x 2 x 0.3 = 1.2 cubic meters which masses 1.2 x 10^3 kilograms.

Weight is a force. f = ma where a is the acceleration of gravity. So it is the same if we write w = mg.

w = 9.8 x 1.2 x 10^3 = 1.176 x 10^4 newtons <-- ANSWER