Speed and Acceleration Question

[azn4lyf89]

A sprinter accelerates to his max speed in 4.0s. He then maintains this speed for the remainder of a 100-m race, finishing with a total time of 9.1s. What is the runner's avg acceleration during the first 4.0s, during the last 5.1s?
I know that the runner starts at 0-m for his initial position and his final position is 100m. I also know that the acceleration from 4.0s-9.0s is 0 because he keeps the same speed. It seems I still have 2 unknown variables when trying to find the position after 4.0s and velocity for each part of the race.
100m=X + V(5.1s) Have 2 unknowns here

 

[edziura]

X + V(5.1) = 100; good start.

Now, assuming acceleration a (from 0 to 4 seconds) is constant, you can write both X and V in terms of a and substitute into your equation. a will then be the only unknown in the equation.