a truck drives 60 m in 5s, while uniformity slowing to a stop. The average speed during this process is 12 m/s. What is the trucks jnitial speed at the beginning of this process? What is the magnitude of the trucks acceleration during this process?

I have the answers I just need to know how to work the problem!

avg speed=12m/s.

avgspeed=(vf+vi)/2=(0+vi)/2=12
vi=24m/s

vf=vi+at
0=24+a5
a= -24/5 m/s^2