Need Help Solving This Really Difficult Quadratic Equation!

Hi all this relates to a Digital Signal Processing question, but it's the maths I can't understand when attempting to solve the quadratic equation.

This was originally part of a transfer function (denominator) and basically reads:

Multiplying this line to positive values of z, to give the quadratic the NOW equation. I'm attempting to solve for r:

a=1

b=

c=

But this where I fall on, and where I need help, because I cannot for the life of me solve with another unknown in it.

all divided by 2

I can change the in the brackets to become = but then I've become snookered. Can anyone help? I can't go any further unfortunatley.

Apologies my Mathtags are terrible! (Can't do the quadratic properly).