1. The problem asks to explain the error in Ella's calculation of the distance between points A and B on a number line, where A is at -1 and B is at 5. 2. The distance between two points on a number line is given by the absolute value of the difference of their coordinates: $$AB = |x_B - x_A|$$ 3. Ella wrote $$AB = |-1 + 5| = 4$$ which is incorrect because she added the coordinates instead of subtracting. 4. The correct calculation is: $$AB = |5 - (-1)| = |5 + 1| = |6| = 6$$ 5. So, the distance between points A and B is 6, not 4. 6. Ella's error was treating the subtraction as addition without considering the negative sign of point A's coordinate. 7. Always remember to subtract the coordinates properly before taking the absolute value to find the distance on a number line.