When constructing a perpendicular line from a point on a line, which step should be performed first?

To construct a perpendicular line from a point on a line, the first step involves placing the needle of a compass on the designated point. This step is crucial because it establishes the center of your arc, which will be essential for determining the points where the perpendicular line intersects the original line.

Once the compass needle is securely placed on the point, you can then move the compass to draw arcs above and below the line. These arcs will intersect the line at two different points. The next steps would typically involve connecting these points to form the perpendicular line.

The other options, while part of the full process, are not the initial action to be taken. For example, drawing an arc on the opposite side or measuring length would occur after establishing where the compass is positioned, and drawing a line parallel to the original line is unrelated to constructing a perpendicular. It's important to follow the steps in a proper order when performing geometric constructions.