Accudraw / Accusnap / moving elements relatively to other elements.

Hi.

I hope somebody knows and can remind me a method I can use to move the blue line parallel to line AB so that the lower end of the blue line ends up at the same elevation as the upper end of the red line. I don't want to use any construction lines just AccuDraw and AccuSnap and tentative snaps.

I knew a way of doing this in the past, but it has escaped my mind now and am unable to figure it out.