If (a,b) , (c,d) and (e,f) are the vertices of a parallelogram the fourth vertex should be (a – c + e , b – d + f) or (c + e – a, d + f - b) or (a + c – e , b + d – f) Since (2,3),(5,1) and (1,–3) are the three vertices of the parallelogram The fourth vertex should be (2 – 5 + 1 , 3 – 1 + ( 3)) = (-2 , -1) OR (5 + 1 – 2 , 1 + ( -3) -3) = (4 , -5) OR (2 + 5 – 1 , 3 + 1-1- (-3)) = (6,7) ∴ (8,1) will not satisfy the above formula. (8,1) will not be the fourth vertex.