On a builder's survey pole, the top of a compound wall is marked +4 m and the bottom of a soak pit is marked −9 m, both measured from ground level (0 m). What is the vertical distance between the two marks?

Count the number of negative factors:
- **Even** number of negatives → product is positive.
- **Odd** number of negatives → product is negative.

Examples: $(-2)\times (-3)\times (-4)$ has three negatives (odd) → negative. Compute the magnitude: $2\times 3\times 4=24$. Result: −24.

$(-2)\times (-3)\times 4$ has two negatives (even) → positive. Magnitude: $2\times 3\times 4=24$. Result: +24.