Lima is constructing a square in which two of its vertices are points a and b. he has already used his straightedge and compass to construct the lines and arcs shown.

The location above point b, label as point C. That distance is the span at which the compass should stay. I am presuming that is is the same distance from A to B in which two sides of the square are consructed. Keeping the span the same, place the point of the compass on point C and create an arc in the vacinity of the third vertex of the square. Repeat this process with the point of the compass on point A without changing the span of the compass. Where the arcs intersect, label point D. Use the straight edge to draw in AD and CD. The square is completed.