Each number on the number line is obtained by subtraction a number equivalent to the length of the given arrows.
In the red arrow points to -3, then you have from the starting point 0:
0 - 3 = -3
The blue arrow points to the position -9 from position -3, then the length of the arrow is 6. Subtract 6 to the previous number -3:
-3 - 6 = -9
Hence, the subtraction expression is - 3 - 6
