There are no doubt other ways, but my Circle Intersect extension (available from the SketchUcation plugin store) was designed to find the exact intersection points of circles to support triangulation problems such as this. Draw one edge - say the 420cm - as a baseline, draw a 150 cm circle at one end and a 400cm circle at the other end. Then use the extension to create guide points at the exact intersections of the circles (the real ones, not the segmented SketchUp representation). Erase the no-longer-needed circles and draw lines from the ends of the baseline to the guide point. If you wish, then erase the guides.
