Solution :
GIven, a rectangle ABCD with vertices A (-3,1) B(-1,3) C (3,-1)
To Calculate the Diagonal, first calculate the length and width of the rectangle.
Length and width of a rectange ABCD are
AB=
BC=

Length and width of rectangle ABCD are
.
Diagonal of the rectangle ABCD =

Hence, the length of a diagonal of a rectangle ABCD with vertices A (-3,1) B(-1,3) C (3,-1) is 6.3