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## Homework Statement

A particle is located at r=(2i+4j)m at t=0s.

At t=3s it is at r=(8i-2j)m and has velocity v=(5i-5j)m/s

a)what is the particles acceleration vector a?

## Homework Equations

r1=r0+v0(t1-t0)+1/2a(t1-t0)^2

v1=v0+at

## The Attempt at a Solution

v1=v0+at

(5i-5j)m/s=v0+a(3s-0s)

v0=(5i-5j)m/s-3a

this is where i have trouble:

I understand how my text uses this equation i just dont know were they got

**these numbers**:

r1=r0+v0(t1-t0)+1/2a(t1-t0)^2

(

**4i**-4j)m=(3s^2)a+

**(2s)**vo

--> did they assume that r0=0? and how did they get r1?

and where did they get 2sv0 because from the information given i thought it was all 3seconds

okay well anyway i understand they would take the v0 found in the first part and sub it in to get this:

(3s^2)a+

**(2s)**[(5i+5j)m/s - 3s(a)] =(4i+4j)m

and a=(2i+2j)m/s^2

Please can someone explain to me where they got the 2 from

and also i dont understand how they subtract3s(a)

I would really appreciate it.