Help needed for finding stopping distance?

1. The problem statement, all variables and given/known data
A tree is being transported on a flatbed trailer by a landscaper. If the base of the tree slides on the trailer, the tree will fall over and be damaged. If the coefficient of static friction between the trailer and the tree is 0.5, what is the minimum stopping distance of the truck, travelling at 15. 278 m/s, if it is to accelerate uniformly and do not have the tree slide forward and fall on the trailer?
Mu s= 0.5
V1= 15.278 m/s
d=?

2. Relevant equations
I'm not sure but i think
v2^2=v1^2+2ad
Us= Ffs/Fn
f=ma
v=d/t

3. The attempt at a solution
i don't really know what to do since i wasn't given the mass so i cant find acceleration or normal force
0.5(9.8)= 4.9
a= -4.9 m/s^2
v2^2-v1^2/2a=d
0^2-15.278^2?29-4.9)
d= 23.8 m

 

how would i find acceleration without the time?
and by using an unknown m do you mean Us= Ffs/ 9.8 x m ?

 

Ffs=UFn
=0.5(9.8m)
=4.9m
i first found the frictional force and then i changed it to the acceleration
a= -4.9 m/s^2
then i plugged it into the first relevant equation
v2^2-v1^2/2a=d
0^2-15.278^2/(2 x -4.9)=d
d= 23.8 m
is this the correct way to do it?

 

also since i found Ffs= - 4.9 m
should i put it into F=ma?
4.9m= ma
a= 4.9m/m
and that way m gets cancelled out?

 

all right got it. so if this was a test question i would need to show the F=ma part? Thanks a lot for your help :)