Answer
$(a)$ See the image below.
$(b)$ $y = 1.84458x - 3318.48042$
Where $y$ is the level of $CO_2$ and $x$ is year.
$(c)$ $ \approx 391.0$
The actual level of $CO_2$ was $0.6$ higher than the value calculated by approximation.
Work Step by Step
$(a)$ See the image above.
$(b)$ We can simply input the data into the regression line calculator and find the function:
$y = 1.84458x - 3318.48042$
Where $y$ is the level of $CO_2$ and $x$ is year.
$(c)$ If $x=2011$
$y = 1.84458\times 2011 - 3318.48042 \approx 391.0$
The actual level of $CO_2$ was $0.6$ higher than the value calculated by approximation.