Use this applet to figure out what the slope is for parallel lines.

Grab the line tool and place the two points to form a line. You should see line "a" made by points "A" and "B".
Use the parallel line tool to create a new line. First place a point then click on Line a. You should see a new parallel line, "b". Add a two more parallel lines.
What do you notice about the equations on the left? Specifically, what are the slopes of lines a, b, c, and d?
Grab the arrow (or the move) tool. Click and move Point B. What happens to the slopes now?
Move Line c. What happens to the slope?