Answers are:
16.
-1 - (-4) = 3
17.
-1-1/3 - (-4) = 2-2/3
18.
2 - (-1) = 3
19.
3-1/2 - (-1) = 4-1/2
20.
3-1/2 - 2 .= 1-1/2
21.
3-1/2 - (-1-1/3) = 4-5/6
Step by step: I took one point and subtracted the second point to get the length of each segment. You could also look at the ruler and count from one point to the other.
Remember when you subtract a negative, the two negatives become a positive