Do you understand what the diagram is showing?

Add this to the program:

CIRCLE (90, 229), 3, 5

This is the point where the line between the eye and the star crosses the screen. Basically you need to find the co-ordinates of this point, after being told the co-ordinates of the star.

The diagram is a side-view of the situation. The red line is the Z axis. The Y axis points upwards from the Eye. The X axis cannot be seen, and any points that share the same X co-ordinate are "flattened" into one. For instance, the screen itself has been squashed; you can tell how high the purple point is on the screen, but not whether it is on the left or right side of the screen.

If the centre of the screen is (0, 0, 1) then the length of the vertical blue line is q! The distance along the red line from the eye to where the red line meets the green line is z. The length of the vertical green line is y.

See if that gets you anywhere.