Given only the information here, no formula will work. I imagine that you are expected to assume that the car slows down with constant acceleration.
If the car slows from 26.0 m/s to rest (0 m/s) in 4.5 s then its acceleration is (0- 26)/4.5= -5.78 m/s^{2} (notice the negative- that important!).
Do you know the formula s= (1/2)at^{2}+ v_{0}t? You were given the time, t, and the initial speed v_{0} and you now know the acceleration, a.
Actually, that's the "hard" way- for this problem you can use the fact that, with a constant acceleration, the "average" speed is just the average of the initial and final speeds. What is the average of 26.0 and 0? If you go at that average speed for 4.5 seconds, how far do you go? Do the problem both ways and compare the answers!
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