Starting with a positive 9, subtracting 4 is the same as adding a negative 4. Subtracting a negative 3 is the same as adding the opposite, which would be positive 3. Representing this on a number line, starting at 0 and going 9 units to the right gives us a positive 9. Then, going 4 units to the left gives us a result of 5. Going 5 units to the left gives us a negative 5, and subtracting 3 units to the left gives us a negative 2. .
Subtracting a number is the same thing as adding its opposite. This is useful because we can even think about doing things with rational numbers, such as negative fractions. For example, 3 minus negative two-fifths is the same thing as adding two-fifths. This can be seen on a number line, where we start with a 3 and move two-fifths to the right.