Answer:
Step-by-step explanation:
It may help you to think of this situation in this manner:
1. The temperature drops 21 degrees from 21 degrees to zero degrees.
2. The temp. then drops 3 degrees from zero to -3 degrees.
3. The total change (which was a drop) was 21 + 3, or 24, degrees.
Summary: from 21 degrees to -3 degrees is a change of 24 degrees.
More explicitly, that's a drop of 24 degrees.
