25 degrees is a constant-- it never changes, no matter how many hours goes by. So, you don't multiply a variable with it.
The temperature drops every 3 hours-- this is not constant, since it's dependent on the number of hours that passes. So, use a variable here. Since the temperature drops every three hours, it's -3, and not 3.
If we know that when , , then we can use a direct proportion equation (). The temperature decreases every hour by 3 degrees. However, we know that before any temperature drop, the temperature was 25 degrees. All together, this gives us , where is the temperature drop each hour, and is the starting temperature. Hope that explained things :)
