In this question, the vectors i and j and unit vectors east and north respectively.
(a) A particle moves in two dimensions with constant acceleration. Initially it has
position vector 3i+2j m relative to a fixed origin and has initial velocity i+j ms-1.
After 4 seconds it has position vector 7i-4j m.
What is its velocity at that time? 
(b) The particle then continues at constant speed.
Calculate how long the particle has been travelling at constant speed when it is southeast of the origin. 
I did part a using s=1/2(u+v)t
For part b, I know that SE i and j components will be equal. How do I start this question?