Question: I dont get the solution with RootOf(_Z)

Hi everyone, Im trying to solve a catenary problem. x is defined in terms of y1, y2, and a. But in my case a is the only unknown. I try to express a in terms of x, y1 and y2 by the following script:

> x := a*arccosh((y1+a)/a)+a*arccosh((y2+a)/a);

                      /y1 + a\                 /y2 + a\
       a arccosh|----------| + a arccosh|--------|
                      \  a   /                    \  a   /

> solve(x, a);

                    /         /y1 + _Z\                /y2 + _Z\\
          RootOf|arccosh|----------| + arccosh|----------||
                    \         \    _Z   /                \  _Z   //  

I dont get the format of this answer. What is _Z? And at last, how can I express a in terms of x, y1 and y2?

I hope you can help me.