The si unit for energy is the joule (j). sam wants to convert 338.25 j into kj.

1 kJ - 1000 J

Conversion:

338.25 J x 1kJ/1000J =  0.33825 kJ

International System of Units (SI Units) are organized body of measurements for physical quantity. They are set to be the norm or global norm scaling for every physical quantity which includes kilogram, meter, second, ampere, kelvin, candela and mole. These measurements then can increase or decrease by the power of ten, multiplied or divided. As said and explained, the SI is helpful in describing objects because 
1.       They give us the idea of how much matter is contained in that single substance or the current state the matter is in or how hot or cold. We measure and can quantify the quality of the specific matter. 
2.       SI Units are a global set of measurement hence, we can communicate with ease from western to eastern countries with these measurements without having problems in terms of portraying or displaying a set of physical quantities.