A local cafe sells $7,200$ cold beverages in the month of July, Over the next six months, the number of cold beverages decreases at an average rate of $750$ cups per month. During the same time frame, the number of hot beverages sold is represented by the equation $y=900x+3,300,$ where the number of hot beverages, $y,$ is a function of $x,$ the number of months after July.

a$.$ What is the rate of change of cold beverages sold per month? hot beverages

b$.$ Which rate of change is greater? What does it mean?