It is easy to compute 6 - 4 = 2 and 14 - 5 = 9, because the answers are positive. But how would one
compute 4 - 6? On the number line, moving to the left is equivalent to subtracting from a number, and
moving to the right is equivalent to adding to a nu mber. To see what 4 - 6 is equal to, start on the
number line at the place marked "4" and count 6 spaces to the left:

Thus, 4 - 6 = - 2.

Notice the similarity between 6 - 4 = 2 and 4 - 6 = - 2. 6 - 4 is the opposite of 4 - 6. To find
4 - 6, simply find 6 - 4 and take its opposite. In fact, this works for any two numbers: to subtract
a larger number from a smaller number, reverse the two numbers and take the opposite of the
answer.
5 - 14 = - (14 - 5) = - 9.

To subtract a positive number from a negative number, switch both signs,
and take the opposite of the answer. -5 - 7 = - (5 + 7) = - 12 and -8 - 11 = - (8 + 11) = - 19.

Adding and Subtracting Negative Numbers

As we just saw, adding 6 to a number is the same
as moving 6 spaces to the right on the number line. Adding -6 to a number is
the same as moving 6 spaces to the left. Thus, adding -6 to a number is the
same as subtracting 6 from the number. In general terms, adding a number is the
same as subtracting its opposite.

Thus, to add a negative number, subtract its opposite. To
subtract a negative number, add its opposite.