The four vertices of the new quadrilateral is the midpoints of the four sides. (5,3) and (5,-1) is ((5+5)/2,(3-1)/2)=(5,1). Midpoint between (-1,3) and (5,3)=(2,3). Midpoint between (-1,3) and (-1,-1) is (-1,1). Midpoint between (-1,-1) and (5,-1) is (2,-1). So the four vertices are (5,1), (2,3), (-1,1), and (2,-1). The resulting figure is a rhombus, centered at (2,1).