a particle leaves the origin with an initial velocity v and constant acceleration a.when the particle reaches its maximium x coordinate,what is its velocity?,acceleration?

The maximum x-coordinate is possible only when the velocity and the constant acceleration(or their x-components) are in opposite directions (for if they have x-component in same direction, there is no 'maximum' defined). In this case, the maximum x-coordinate is reached when the Vx becomes zero and the motion turns its direction in -ve x direction. The velocity is only the y- comp. of velocity, (increased or decreased) coming as per y-component of acceleration(for motion in x-direction only in initial case, this is also zero, "so in unidirectional motion the velocity is zero"). And whatever the case may be, the acceleration is always 'a' as it is a constant, stated earlier. Thanks