In the equation of the trend line you provided, \( y = -7.2x + 98.9 \), the slope is \(-7.2\).
The slope represents the rate of change of the average final grade (\(y\)) with respect to the number of hours spent on social media (\(x\)). In this case, a slope of \(-7.2\) means that for each additional hour spent on social media, the average final grade decreases by 7.2 points.
So, to answer your question directly, the average final grade decreases by 7.2 points for each additional hour spent on social media.