I'm having a little trouble with a question I'm doing. I'll write it below.

The vertices of a quadrilateral are C(-2,4), D(3,9), E(6,6), and F(1,1).

a) Show that the quadrilateral is a rectangle.

First I calculated the length of its four sides.

CD = square root of 50

DE = square root of 18

EF = square root of 50

CF = square root of 18

Ok, so far so good. Now I have to show that it has a right angle, which would require two adjacent sides being perpendicular.

mCD = 1, but mDE =-6/0?

Did I mess up somewhere? Any help/advice is appreciated.