In principle, this is the right construction. However, you did not perform a proper construction, as can be seen by the two tiny red circles: Instead of constru

In principle, this is the right construction. However, you did not perform a proper construction, as can be seen by the two tiny red circles: Instead of constructing a circle passing through the end points of the given line, you "eyeballed" the circle diameter. Which may look right at this resolution, but if you were to zoom in sufficiently, you would inevitable end up with a circle that oh-so-slightly misses to go through one of the end points of the base line. In the smartphone app, to do this right you need to select the circle tool, start at one end point of the line, then drag your finger exactly to the other end point -- it'll "snap" to that, and create a circle through the second end point.