To predict the average final grade percentage when a student spends 10 hours on social media, we can substitute \( x = 10 \) into the equation of the trend line:
\[ y = -7.2x + 98.9 \]
Substituting \( x = 10 \):
\[ y = -7.2(10) + 98.9 \] \[ y = -72 + 98.9 \] \[ y = 26.9 \]
Rounding this to the nearest whole number, we get:
\[ y \approx 27 \]
Therefore, the predicted average final grade percentage, if a student spent 10 hours on social media, is 27%.