6.77:​
​
a. 2.39 m/s in the direction towards the wall
b. a = 9.42 m/s^2 away from the wall
6.81:
The coefficient of kinetic friction is 0.79.
6.72:
a.​ 2.41*10^5
b.​ 2.82 *10^-10 m
c.​ 3 *10^5 m/s