Answer
"number of rabbits / day" .
"number of foxes / day" .
Work Step by Step
Step 1. For the rabbits, the vertical unit is the number of rabbits and the horizontal unit is the number of days; thus the unit for the derivative should be "number of rabbits / day" .
Step 2. For the foxes, the vertical unit is the number of foxes and the horizontal unit is the number of days; thus the unit for the derivative should be "number of foxes / day" .