To find the horizontal acceleration when given vertical acceleration and velocity, we need to understand that the horizontal and vertical motions of an object are independent of each other in ideal projectile motion (assuming no air resistance). In other words, the horizontal motion does not affect the vertical motion and vice versa.
Therefore, the horizontal acceleration is zero in this scenario. Regardless of the values of the vertical acceleration and velocity, the horizontal acceleration remains constant at zero throughout the motion.
This means that the object's horizontal velocity remains constant throughout its flight, assuming no external forces act upon it. The only force acting on the object in the horizontal direction is its initial horizontal velocity, which remains unchanged unless acted upon by an external force.