Circle Tangent to a Line and a Circle
The completed construction looks like the applet below. Drag point E along the line. Change the size of the grey circle by dragging point D or C. The large circle is tangent to both the line and the grey circle.
The center G traces a parabola. Where is the focus? Where is the directrix?
You Try It!
Try the construction on your own! What parts are giving you trouble? At some point in the construction, you will need to use the COMPASS TOOL 
Still not sure?
Play through the construction below!