Rule: y=x-3. like i don’t rly know how i’m supposed to do this one i have all the other ones done and i’m stuck

Sanya noticed that the temperature was falling at a steady rate of 1.4 degrees every hour from the time that she first checked her outdoor thermometer. by 6 a.m., the temperature had fallen 21 degrees. which expression can you use to find how many hours earlier she had first checked the thermometer?