For the edification of those who don’t already know (if you already know or don’t care, skip to another topic ):

If you wanted to solve a problem like this using pencil and paper, you would draw the bottom and left sides, swing arcs of the required radii with a compass from their endpoints, and draw the lines to the point where the arcs cross.

In SketchUp the analagous process isn’t reliable because SketchUp represents an arc as a sequence of straight segments. Only the endpoints of these segments are at the target radius of the arc. So, only if endpoints from both arcs just happen to land at the same place will their crossing point in SketchUp be the exact answer (a very remote likelihood). There are several extensions for the desktop version of SketchUp that address this exact problem, but extensions are not yet possible for the web version.

The process @DaveR showed works because he ended the arc on the target line, and the end of an arc (like all its vertices) is at the exact radius. But you can’t do this when both lines can be pivoted.