Solving for inductance L in an AC series+parallel circuit is .. quartic?
Since you know both the source voltage and the voltage across the divider network, you can ignore phase and just use the magnitudes of the voltages and impedances. The magnitude of the parallel combination of the L and R can be found from the expression you had by dividing the magnitude of the numerator by the magnitude of the denominator.  This will result in a much simplified expression than what you did.  Apply that expression in the voltage divider equation.  Multiply it out and place the term with the square root on one side and all the other terms on the other side.  If you then square both sides, you will wind up with a quadratic equation for L