The formula for acceleration is (final velocity - starting velocity) / time to change velocities.
Plug in your values into the formula, and make sure to follow the correct order of operations.
Starting velocity = 18 m/s
Final velocity = 60 m/s
Time it took to change velocities = 12 seconds.
Acceleration = (60 - 18) / 12
Acceleration = 42 / 12
Acceleration = 3.5 m/s/s
The unit for acceleration looks a bit goofy. What the unit is telling you is that the person accelerated by 3.5 m/s every single second until the final speed was reached. Sometimes it is helpful to think about the unit with miles per hour. 3.5 mi/hr/sec means that the person accelerated by a speed of 3.5 mi/hr every single second until the final speed was reached.