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Find an equation for the linear function which has slope -10 and x-intercept -7

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?

9/10 divide by -3/15 as a mixed number