To calculate the acceleration between 6 and 9 seconds, you need to determine the change in velocity during that time interval. Here's how you can do it:

1. Determine the initial velocity at 6 seconds. Let's call it v1.

2. Determine the final velocity at 9 seconds. Let's call it v2. Make sure that both v1 and v2 are measured in the same units (e.g., meters per second).

3. Calculate the change in velocity (Δv) by subtracting v1 from v2: Δv = v2 - v1.

4. Calculate the elapsed time interval (Δt) by subtracting the initial time (6 seconds) from the final time (9 seconds): Δt = 9 seconds - 6 seconds = 3 seconds.

5. Calculate the acceleration (a) by dividing the change in velocity (Δv) by the elapsed time interval (Δt): a = Δv/Δt.

The result will be the acceleration between 6 and 9 seconds in the unit meters per second squared.