We can solve this problem by problem!
Exercise 21
In this exercise, we find the initial math to subtract to -3.5. The error here is that length cannot be negative. We want to find the length of AB, so we can subtract with the expression in a modulus. This would look like:
|1 - 4.5| = 3.5
Therefore, the length of segment AB is 3.5 cm.
Exercise 22
In this exercise, we are given a simple addition problem of 1 + 4.5. Because the numbers already are positive, no modulus is needed to give us a result of 5.5 cm; however, this is the incorrect length. To solve for the length, we can note that the length of AB = the length of BA. With this in mind, we must identify the difference of point B and point A:
4.5 - 1 = 3.5
Hence, the length of line AB = 3.5 cm.