Being completly honest, I am not good with these things, but I'll give it my best shot.
Let's start by turning everything into sine/cosine.
(cos θ/sin θ) - (1/sin θ^2)/(cos θ/sin θ) = -(sine θ/cos θ)
(cos θ/sin θ) - (1/(sin θ * cos θ)) = -(sine θ/cos θ)
(cos θ^2 - 1)/(sin θ * cos θ) = -(sine θ/cos θ)
(cos θ^2 - 1) = (sin θ * cos θ) * -(sine θ/cos θ)
(cos θ^2 - 1) = - (sin θ^2)
cos θ = sqrt(1-sine θ^2)
This equation can be proven by using the unit circle and pythagreon theorum.
Now, let's plug that into the original equation.
(sqrt(1-sine θ^2)^2 - 1) = - (sin θ^2)
(1-sine θ^2 - 1) = - (sin θ^2)
- sine θ^2 = - (sin θ^2)
- sine θ^2 = - sine θ^2
omgosh, I can't believe I solved that. (hopefully it's correct)
I hope this helps. :)))
=^._.^=