To calculate Matias's marginal cost in terms of TV viewing to move from a B to an A on the exam, we need to determine the additional time he spent studying beyond what would have been required to achieve a B.
Matias normally would have spent the entire 6 hours watching TV instead of studying. However, to achieve a B, he would have only needed to study for 3 hours. This means that the additional time he spent studying to move from a B to an A is 6 hours - 3 hours = 3 hours.
Therefore, Matias's marginal cost in terms of TV viewing to move from a B to an A on the exam is 3 hours of TV viewing.
I hope this helps! :)