The acceleration is defined as the ratio between the change in velocity and the time elapsed to perform such a change.
These "changes" are indicated with the capital greek letter delta, , and when you write
you mean the difference between the finial and the inital values of the variable x:
So, the acceleration is defined as
In this case, the initial velocity is 35, the final velocity is 65. Assuming we start the clock at the beginning of the observation, the inital time is 0 and the final time is 5. So, we have
m/s^2