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.