Question

The driver of a truck slams on the brakes when he sees a tree blocking the road. The truck slows down uniformly with acceleration
−6.1 m/s2
for 4.20 s, making skid marks 62.5 m long that end at the tree. With what speed does the truck then strike the tree

Answer 1

you have 2 unknowns - the initial speed, u, and the final speed v

so you need 2 equations to solve it

use

s= ut +0.5at^2
to sulve for u

then use
v^2= u^2 +2as

to solve for v