Net cash flow is basically the difference of the cash balance from the beginning of the period to the end of the period. For this instance, we take sales and subtract the listed expenses.
January = 150,000 - 35,000- 20,000 -20,000 = 75,000 net cash flow
February = 175,000 - 39,000 - 25,000 - 45,000 = 66,000 net cash flow
For the change you divide (February/January) -1 or (66,000/75,000)-1= -.12
The growth in cash flow was -12%