Once the football has left the thrower's hand, it's turn in to a projectile. which means that it is not accelerating horizontally. The only time the ball is accelerating horizontally is when the thrower is apply a force on to the football. because it is an unbalanced force acting on an object. This force is an unbalanced force becuse there is no other force pushing back at it the other way to stop the ball form accelerating. When the ball is in the air, there is nothing pushing it horizontally forwards or backwards, because there's no air resistance. So we know that it is no longer being accelerate horizontally. The only force that's acting on the football when is the force of gravity which accelerate the football at a rate of 9.8m/s^2 downward. We will need to find the horizontal and vertical components of the acceleration first, then use Pythagorean theorem to find the actual acceleration.