d = (r/1+rt)
r = (d/1-td)
I'm not sure why you enclosed the right side of your equation in parentheses. And your right side is a bit ambiguous. Is it
I'll assume it is and proceed from there.
First, multiply both sides of your equation by 1 + rt
Next, transpose drt to the right side.
Factor an r out of the right side.
Finally, divide both sides by 1 - dt