That equation involves x both "inside" and "outside" a transcendental function. There is no standard "algebraic" method of solving it and I suspect there is no "standard" notation for writing the number that is its solution. It reduces, of course, to x= cot(x). I'm not sure what you mean by "guessing" but graphing would be one way of getting an approximate solution. There are also methods for numerically getting approximate solutions to any desired accuracy, by "bisection" or "Newton's method", say.