A technology specialist studied the relationship between time spent on social media and age. One observation found that a 40-year-old person spends 12 hour per week on social media. A second observation showed that a 25-year-old person spends 18 hours per week on social media. 

Assuming time spent per week on social media T (in hr) is a linear function of age a (in yr), find a linear model of this event.

Use the model to find the age (in yr) of a person who spends 10 hours per week on social media.