1. Calculate the velocity after 6 seconds:

* Initial velocity (v₀) = 0 m/s (at rest)

* Acceleration (a) = 1.7 m/s²

* Time (t) = 6 s

* Final velocity (v) = v₀ + at = 0 + (1.7 m/s²)(6 s) = 10.2 m/s

2. Calculate the total distance traveled:

* Distance traveled during acceleration (d₁) = (1/2)at² = (1/2)(1.7 m/s²)(6 s)² = 30.6 m

* Distance traveled at constant velocity (d₂) = v * t = (10.2 m/s)(9 s) = 91.8 m

* Total distance (d) = d₁ + d₂ = 30.6 m + 91.8 m = 122.4 m

3. Calculate the average velocity:

* Average velocity (v_avg) = Total distance / Total time = 122.4 m / 15 s = 8.16 m/s

Therefore, the object's average velocity over the 15-second interval is 8.16 m/s.