Angry Birds Parabola

Your task is to come up with three quadratic equations that will create parabolas that goes through both the Angry Bird on the slingshot and the Pig. At least one of these parabolas must stay on the screen at all times.
Why do you think the a value must always be negative in this situation? For one of your parabolas that is always on the screen, note the vertex and axis of symmetry. For a parabola that goes out of the top of the screen, can you think of a way to work out how high it goes at it's highest point?