polylines

Have you tried the help file?
here's my text screen of that exercise ( pline width was set to .5 when I started, and I was zoomed in too close):
Command: pl PLINE From point: 100,100
Current line-width is 0.0500 Arc/Close/Halfwidth/Length/Undo/Width/: w
Starting width : 0
Ending width : 0
Arc/Close/Halfwidth/Length/Undo/Width/: 100,150
Arc/Close/Halfwidth/Length/Undo/Width/: 200,150
Arc/Close/Halfwidth/Length/Undo/Width/: 200,200
Arc/Close/Halfwidth/Length/Undo/Width/: -100,200
Arc/Close/Halfwidth/Length/Undo/Width/: 'z
Resuming PLINE command.
Arc/Close/Halfwidth/Length/Undo/Width/: a
Angle/CEnter/CLose/Direction/Halfwidth/Line/Radius/Second pt/Undo/Width/ : ce
Center point: 50,50
Angle/Length/: a Included angle: 90
Angle/CEnter/CLose/Direction/Halfwidth/Line/Radius/Second pt/Undo/Width/ : (Enter to exit command)