f(x) = (ax+b)/(cx+d) , x is real, x=/= -d/c , b=/=0 , c=/=0

Prove that if

a + d =/= 0

and

(a-d)^2 +4bc = 0

then y=f(x) and y=f^-1(x) intersect at exactly one point.

This is what I did:

f(x) = f^-1(x)

(ax+b)/(cx+d) = (b-dx)/(cx-a)

(ax+b)(cx-a) = (b-dx)(cx+d)

then all I can think to do is write as a polynamial equal to zero, and find the discriminant. This didn't get me (a-d)^2 +4bc.

Is this method correct or am I missing something? Is there another approach?