In the shape to the right prove whether or not there are any

In the shape to the right, prove whether or not there are any perpendicular lines.

A= ( -4 , 2.5) ; B = ( 2, 4) ; C = ( 3, 1)

D = ( -3 , -1)

Find slopes between the points

AB = ( 4- 2.5)/(2 +4) = 1.5/6 = 15/60 = 5/20 = 1/4

BC = ( 1 -4)/(3 -2) = -3

CD = (-1 -1)/(-3-3) = 1/3

AD = (-3.5/1) = -3.5 = -7/2

Only sides BC and AD are perpendicular as -3*(1/3) = -1