Physics
posted by Christian .
A particle has ⃗r(0) = (4 m)j and ⃗v(0) = (2 m/s)i.
If its acceleration is constant and given by ⃗a = −(2 m/s2) (i +j), at what time t does the particle first cross the x axis?
Answer in units of s.
part 2. At what time t is the particle moving parallel to the y axis; that is, in the ˆ direction?
Answer in units of s.

part 2. At what time t is the particle moving parallel to the y axis; that is, in the j direction?
Answer in units of s.
im really lost and if anyone could atleast tell me which equation to use it would be greatly appreciated