Question

this is true for any integers a and b. (it is actually true for any numbers a and b as well, as we will see in later chapters.)
we already knew how to subtract with whole numbers. lets check to make sure that our new definition agrees with our understanding of subtraction for whole numbers. for example, 7 - 4 = 3 for whole numbers now becomes 7+(-4) = 3. and so our new understanding of subtraction agrees with our old understanding but it also allows us to subtract negative integers.
keeping in mind what the opposite of an integer means. apply this rule to the problems that follow:
now try this problem:
suppose you want to subtract these two numbers: 7 - 9
re - write this subtraction as an addition of signed numbers: 7 - 9=
now actually compute the result: 7 - 9=
question help: worked example 1 message instructor

Explanation:

Step1: Rewrite subtraction as addition

Subtracting a number is the same as adding its opposite. So, $7 - 9$ can be rewritten as $7+(- 9)$.

Step2: Compute the sum

When adding a positive and a negative number, we subtract the absolute - values and take the sign of the number with the larger absolute value. $|7| = 7$ and $|-9|=9$. Since $9>7$, the result is negative. $9 - 7=2$, so $7+(-9)=-2$.

Answer:

First blank: $7+(-9)$
Second blank: $-2$