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