If you speed up from rest to 12m/s in 3 seconds, what is your acceleration?
a.) 4m
b.) 4m/s/s
c.) 36m
d.) 36m/s

b) 4m/s/s

This is because you divide the speed you reach, by the time it takes to get to that speed:

12m/s ÷ 3s = 4m/s/s

The units come from what you divide, meters per second ÷ seconds this can be written as m/s/s or ms-² 

neither accurate nor precise.