How Do You Calculate Frictional Force in a Braking Scenario?

[Lami]

I'm very new to physics and am doing a crash higher course. I am currently tackling dynamics, and here is one of the questions I was given.
8 ) A car of mass 800kg is accelerated from rest to 18m/s in 12 seconds.
a) What is the size of the resultant force acting on the car
b) How far does it travel in these 12 seconds?
c) At the end of 12 seconds period the brakes are operated and the car comes to rest in a distance of 50m. What is the size of the average frictional force acting on the car?

I got for a) F=ma =800 x 1.5 = 1200N For b) s= ut+ 1/2at^2 = 0.5 x 1.5 x 144 = 108m. I'm confused with how to go about tackling c and was wondering how to tackle it? Apologies in advance for if it's blarringly obvious.

 

[bsheikho]

you have distance, initial velocity, final velocity and solving for acceleration. Solve the kinematic equation that doesn't have time.

 

[cnh1995]

I'm confused with how to go about tackling c and was wondering how to tackle it?

What is the final velocity in part c? What is the velocity at the instant of application of brakes? Can you calculate the deceleration from this?