Finding angular frequency in electric circuit

[sodper]

Homework Statement

At a certain angular frequency, the phase difference between U_{2} and U_{1} is 180^{\circ}.
a) Calculate this angular frequency
b) Calculate U_{2} at this angular frequency

See the attachment for circuit configuration.

Homework Equations

Ohms law: U = Z \cdot I
Voltage: U = \hat{U} cos(\omega t + \alpha)

The Attempt at a Solution

I can't seem to find the relation between phase difference and angular frequency.

I've tried to compute the equivalent double-pole, separating the coil with voltage U_2 from the rest och the circuit, with the following result:

Idle current:
U_t = \frac{\omega^2 L^2 + Rj\omega L}{R^2 + \omega^2 L^2} U_1

Equivalent impedance:
Z_0 = \frac{\omega^2 RCL - R - j\omega L}{\omega^2 LC - j\omega RC}

And from this I've calculated U_{2}:
U_{2} = \frac{j\omega L}{Z_0 + j\omega L} U_t

But as stated, I can't find the relation between angular frequency and phase difference. How do I use the phase difference?

 

[sodper]

I'm stuck here. Does anyone have a suggestion?