The commonly accepted values is 4 Calories per gram of protein and carbohydrate and 9 Calories per gram of fat. Since we have 2 grams of each, a little simple math will do:
2 g * 4 Cal/g + 2 g * 4 Cal/g + 2 g *9 Cal/g = 8 Cal + 8 Cal + 18 Cal = 34 Cal
