In answering this problem, you have to know the concepts behind Newton's Laws of Motion. When a coin is tossed up in the air, the only force acting upon it is the force of gravity. It is the force pulling the object towards the center of the Earth. This acceleration, according to Newton's 2nd Law of Motion, produces a constant acceleration equal to 9.81 m/s². This is an empirical data that is specific only to circumstances on Earth.
Part a. From the starting point, you exert a force so it speeds up with an initial velocity. As it goes up, the velocity decreases because the force of gravity is constantly pulling it down. The coin will eventually reach a maximum height, where its velocity will reach zero. Afterwhich, the velocity increases again as it goes back down to the ground. The motion is now faster because it is parallel and in the same direction of the gravitational force.
Part b. As mentioned earlier, the acceleration is constant because the force of gravity is always acting upon an object. Whether it goes up and returns back down, the acceleration will always be of the same value.
