Accuracy is how close a measured item is to the actual or true value and precision is how close the measured items are to being the same. So you could weight 5 apples. If each apple weights 2 lbs and you said that the 5 apples weight a total of 10lbs you are determining the precision. Precision is independent of accuracy. Accuracy would be if you put all 5 apples on the scale at one time and knew a exact amount.
If you have -300 and add +300 you will end up with 0 because when a positive and a negative that are equal cancel each other out. So if you add +300 to -300 you get 0 but its 400 so 300 + 100 is 400 so you have +100 left. : )
is 5(x+3y) and the best answer for LCD is 
is 2(x+3y)