Step 1:
Find the greatest common factor of -45x and +72.
3 | -45x | +72
-----------------------------------------
3 | -15x | +24
-----------------------------------------
| -5x | +8
Therefore, the greatest common factor of -45x and +72 is 3 x 3 = 9
Step 2:
Factor out the greatest common factor
Therefore the correct answer 9(-5x + 8)