The simplest way to draw the circled Line, if from lower left to upper right, is by giving it the start point and then:
@10,10
i.e. a relative X,Y displacement [with running Osnap turned off, of course]. Then you don't need to know the length [which will be irrational, involving the square root of 2], as you would to get it right with @distance<45. If you do it that way with an approximate distance, you'll need to then Fillet it with the upper horizontal Line to get the right length.
Kent Cooper, AIA