How do i work out substitution problems like this?

The difference between tax advoidance and tax evasion

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?

What are the factors of me - 12m + 20?