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.
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