Data on tuition and mid-career salary are collected from a number of universities and colleges. The result of the data collection is the linear regression model

$\displaystyle \hat{{y}}=-{0.98}{x}+{155}$

where $\displaystyle {x}=$annual tuition and $\displaystyle {y}=$average mid-career salary of graduates, both in thousands of dollars.

1. Which quantity is the independent variable?
   • annual tuition
   • average mid-career salary of graduates
   
   
2. According to this model, what is the average salary for a graduate of a college or university where the annual tuition is $30,000?
   $
3. What is the slope of this regression model?