How to find rate of change on a table

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?

Answer pls man im trying to get out of summer school

Which of the following is the best first step in solving the equation below? 4+2㏒³x=17