1)  When writing out a velocity or acceleration, remember to note the direction.  If your car rolled toward the back of the school (the
cafeteria), that is south. If your car rolled toward the front of the school (main entrance), that is north.

2)  Here is a sample data table, yours should be similar.  This is all false data, so don't expect to have the same numbers. For 3 trials:
 
 
 

Elapsed time  (seconds)	Distance traveled  (centimeters)	Average distance  (centimeters)	Velocity  (cm/sec, N or S)
0
1	50, 51, 51	51	51
2	104, 105,106	105	54
3	127, 128, 129	128	23
4	143, 144, 145	144	16

Remember to calculate velocity for each second.

3)  For average velocity, look at your graph and find the data point that represents the maximum time and maximum distance, then calculate.

4)  For acceleration calculations, here are 2 examples based on the data listed above.

 For 0 - 2 seconds 53 cm/sec - 0 cm/sec   =  27 cm/sec2  North or 27 cm/sec/sec
     2 seconds - 0 seconds    North

 For 2 - 4 seconds 32 cm/sec - 53 cm/sec   =  -11 cm/sec2  North  or
     4 seconds - 2 seconds                            -11 cm/sec/sec  North

Notice that 2-4 seconds comes out as a negative number.  If this car was rolled to the north, then my acceleration for 2-4 seconds could be written
as:  -11 cm/sec/sec North   OR  11 cm/sec/sec South

Also, cm/sec/sec can be written as cm/sec2