The points $(1,1),(-1,5),(7,9)$ and  $(9,5)$ taken in such order that it will form a 

Let the points be $A(1,1),B(-1,5),C(7,9)$ and $D(9,5)$

Now, by using distance formula, we get

Hence, $AB=CD$ and $BC=DA$ and $AC=BD$

$\therefore ABCD$  forms a rectangle.