How do you subtract the calories for key nutrients contained by food?Please show your work.?

Actually, I believe it's measured using a piece of equipment called a bomb calorimeter.
It requires placing a taste of the food in an insulated chamber, burning it next to a known amount of enthusiasm, and measuring the increase contained by temperature.
If the food contained 0 calories (energy), it would release no supplementary energy. If the food contained calories, this would do an additional increase surrounded by temperature, which can be converted into calories.