A motorcyclist driving east through a small town accelerates after passing a sig

Question

A motorcyclist driving east through a small town accelerates after passing a signpost
at x = 0 marking the city limits. His acceleration is constant at a = 4.0 m/s2. At time
t = 0 he is 5.0 m east of the signpost and has a velocity of 15 m/s.
(a) What is his position and velocity at time t = 5.0 s?
(b) Where is the motorcyclist when his velocity is 25 m/s?

Can someone explain this a little better than the previous answer for this question.  I don't understant the formula.  And for answer a), is the position and velocity the same for the answer?  Thank you very much.  

Explanation / Answer

from laws of motion

s=u*t+a*t^2/2

=15*5+4*5*5/2
=125 m

as 5 is his initial buffer distance

we need to add it to125

so is

a)position=5+125
=130

again from laws of motion

b)v^2=u^2+2*a*s
25^2=15^2+2*4*s
s=50

add again 5 to this
position=55

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