GDI : How to delete a line on right click

Question

I have a homework, its aplication for drawing lines on the form. I know how to draw them, but I also have to delete them on right click. For example I click on the form with left click, drag a line, on release paint the line, but if I press right click on it, line will be removed. Does anyone have a any clue how to delete lines ???

Please help, I have 5 days left :(