FYI - many of us cannot open Prime 3.1 files, so it's always good to post an image or PDF of your worksheet (along with the worksheet for those who can open).
First, note that "arcsin" is not a built-in function. "asin" is the built-in function. Perhaps the image of your equation is not in Mathcad, or maybe you defined a custom function "arcsin".
I don't think you're going to find a symbolic solution to your problem, but there are a few ways to solve it numerically.