Write the equations of motion in the vertical (y) and horizontal (x) directions for the red ball in the boxes below. Your equation will look something like:

x + v*t + 0.5*a*t^2
x=
y=

Then click on the button to make the blue ball follow your equation of motion. When you have the correct relationship, you will see only the blue ball, since it will be moving exactly on top of the red one. If the two balls do not move together, check you calculations of initial position, velocity and acceleration of the red ball.