1. The problem involves finding the length of a rectangle placed on a number line with its left side at 1 and right side at 3. 2. To find the length of the rectangle, we use the formula for the distance between two points on a number line: $$\text{Length} = |x_2 - x_1|$$ where $x_1$ and $x_2$ are the coordinates of the left and right sides respectively. 3. Substitute the given values: $$\text{Length} = |3 - 1|$$ 4. Calculate the difference: $$\text{Length} = 2$$ 5. Therefore, the length of the rectangle is 2 units. This means the rectangle spans 2 units along the number line from 1 to 3.