For a concave mirror, the radius of curvature is twice the focal length of the mirror:

where f, for a concave mirror, is taken to be positive.
Re-arranging the formula we get:

and since the radius of curvature of the mirror in the problem is 24 cm, the focal length is
Answer:
Acceleration: 
Final velocity: 
Explanation:
To calculate the acceleration, you start from the formula:

Thus:

Now that you have the accelration, you can use the this formula to calculate the final speed:

Thus:

If the person if slowing down, then this would mean that this would actually be "negative acceleration". Sense this person first did a mile in 10 minutes, and then the next mile that this person did was actually 15 minutes, then this shows you that they actually went a lot slower. And this is why this would be a great example of "negative acceleration".