The estimated regression line is: \(\hat{y}=12.53-0.22x.\)

Intercept: \(\hat{\beta}_0=12.53\). A car that has fuel economy of \(0\) mpg in city driving conditions can expect an average carbon footprint of \(12.53\) tonnes per year. The interpretation of the intercept requires that a value of \(x=0\) makes sense. But even when \(x=0\) does not make sense, the intercept is an important part of the model.

Slope: \(\hat{\beta}_1=-0.22\). For every extra mile per gallon, a car's carbon footprint will decrease on average by 0.22 tonnes per year. Alternatively, if the fuel economies of two cars differ by 1 mpg in city driving conditions, their carbon footprints will differ, on average, by 0.22 tonnes per year.