physics
posted by Steve .
A car comes to a stop for a stop sign in 4.6 seconds. If it was initially traveling at 27.5 m/s, how much space did it take for the car to stop?

Vf=Vi+at solve for a.
then
Vf^2=Vi^2 + 2ad solve for d. 
what is Vi?

vi=27.5m/s

this is where get stuck
vf=27.5(a)(4.6)
vf=126.5
i don't know the rest 
please help me im realy stuck