A particle leaves its orgin with a velocity of 4 i m/s and a constant acceleration of (-2i + .5j) m/s^2. At the time the particle reaches its maximum X corrdinate a) what is its velocity? b) what is its position vector?

Question

I'm not sure which equations i should use.

solve for the max x coordinate:

dx= vi*t +1/2 at^2
dx/dt=0=vi + at
t= -vi/a

now, solve for velocity and position at that time.