Afull lake begins dropping at a constant rate. after 4 weeks it has dropped 3 feet. what is the unit rate of change in the lake's level compared to it's full level?

So it is dropping at 3 feet per every 4 weeks. Since the rate is unchanging, you can simplify this to 3/4 (or .75) feet per week. I'm not really sure what you mean by unit rate of change, but you can simplify this further to about .107 feet/day (by dividing .75 by 7).