This is in fact more of a math question than a Mathematica one.
Your equation is one of the many transcendental equations (as opposed to being an algebraic equation that can be solved with methods for solving polynomials) that one cannot solve with the usual ways taught in undergraduate algebra/precalculus courses. In particular, the difficulty in solving this transcendental equation is that your
P is inside a square root, outside a square root, and inside a square root inside a tangent (and the tangent is what makes this equation transcendental).
Mathematica will also be hard-pressed to solve this symbolically, so don't hope for an explicit solution.
Thus, you might have to content yourself with using
FindRoot for a numerical solution. You might wish to plot the left hand side of your equation with specific values of your other parameters to assist in searching for this equation's roots.
As an aside,
K is a reserved
System` symbol; consider using a different name for that parameter.