```
acceleration = (final velocity - initial velocity) / time
```
In this case, the initial velocity is 6 m/s, the final velocity is 2 m/s, and the time is 2 seconds. Plugging these values into the formula, we get:
```
acceleration = (2 m/s - 6 m/s) / 2 s
```
```
acceleration = -4 m/s²
```
Therefore, the runner's acceleration was -4 m/s². The negative sign indicates that the runner's velocity was decreasing, which is consistent with the fact that the runner was slowing down.
