Verify that parallelogram ABCD with vertices A(–5, –1), B(–9, 6), C(–1, 5), and D(3, –2) is a rhombus by showing that it is a parallelogram with perpendicular diagonals.I can't figure this out, but I...

Verify that parallelogram ABCD with vertices A(–5, –1), B(–9, 6),
C(–1, 5), and D(3, –2) is a rhombus by showing that it is a
parallelogram with perpendicular diagonals. I can't figure this out, but
I need to show my work on my homework sheet, and there's no way
for me to graph it. Is there a formula I use?
ANSWER:
Slope of diagonal AC (m1) = (y3-y1)/(x3-x1)= (5-(-1))/(-1-(-5)) =6/4
=3/2
Slope of diagonal BD (m2) = (y4-y2)/(x4-x2)= ((-2)-6)/ (3-(-9)) = -8/12
= -2/3
Product...