A driver is going 29.9 meters per second. After 11 minutes the driver sees the police and hits the brakes on the car slowing way down to 24.59 meters per second. He continues this speed for 9 minutes. He goes in the fast lane which is 26.82 meters per second.

What is the average acceleration as if he were traveling in a straight line?

I think you don't have to consider the 'middle' lapse of time where the driver has a speed of 24.59 m/s.

So, you use the formula

where a is the acceleration, v the final velocity, u the initial velocity and t the time.

Now, using that acceleration over the same lapse of time will get the car from 29.9 m/s to 26.82 m/s.