Slopes of Parallel Lines 平行綫的斜率

http://tube.geogebra.org/material/show/id/177015 (Download GeoGebra File)

Drag the red point to make the two lines parallel.

What is the relation of the slopes of parallel lines?

http://tube.geogebra.org/material/show/id/177009 (Download GeoGebra File)

拖拉紅點，令兩綫平行。

平行綫的斜率有甚麼關係？