Given:
Initial velocity = 11 m/s
Average acceleration = -1.81 m/s²
Let's determine how long it takes the horse to decrease its velocity to 6.5 m/s.
Apply the equation of motion:

Where:
v is the final velocity = 6.5 m/s
u is the initial velocity = 11 m/s
a is the acceleration = -1.81 m/s²
t is the time in seconds.
Thus, we have:

Let's solve for t.
Subtract 11 from both sides:

Therefore, it took the horse approximately 2.49 seconds to decrease its velocity to 6.5 m/s
ANSWER:
2.49 seconds.