Because in the real world, if you have two cars traveling at some speed and you take their average, how can the average be zero? So that's where the root mean square formula comes from. What this formula does is it takes away these negatives and gives you the value of this type of average without the negatives. So let's look at the formula. VRS is equal to, you take the squares of every single velocity or every single value and then you divide that by n. So almost the same thing as you did here, except you square every value. The reason you square every value is because a square will take away that negative sign and then you divide by n and you take the square root of that.