“Two minuses make a plus” is easy to repeat and easy to apply in the wrong place. A number line gives a learner something more useful: a way to check what the subtraction is asking.
Begin with subtraction as a missing addition. If 7 minus 3 equals 4, then 3 plus 4 equals 7. The subtraction tells us the directed change needed to get from the second number to the first. Keep that same relationship when a negative number appears.
Try 3 minus negative 2
Draw a line with evenly spaced marks from negative 4 to positive 5. Put a finger on negative 2. Ask how far, and in which direction, you must move to reach 3. Moving five units to the right gives a change of positive 5.
The addition check is negative 2 plus 5 equals 3. Therefore, 3 minus negative 2 equals 5. Nothing about the meaning of subtraction has been replaced by a special trick.
Now compare 3 minus 2. Start at positive 2 and move one unit right to reach 3. The result is 1. The second number matters because it changes the starting point for this distance interpretation.
Let the learner explain a contrasting pair
- Negative 1 minus negative 4: from negative 4 to negative 1 is a change of positive 3.
- Negative 4 minus negative 1: from negative 1 to negative 4 is a change of negative 3.
Ask the learner to draw the arrows and check each answer with addition. Reversing the order changes the direction, even though the distance between the two marks stays the same.
Keep the signs visually distinct when writing examples. In 3 − (−2), the outside sign names the operation; the sign inside the brackets belongs to the number. Reading it aloud as “three subtract negative two” can help preserve that distinction.
Once the picture is secure, introduce the general rule: subtracting a number is equivalent to adding its opposite. The rule becomes a compact description of reasoning the learner has already tested.