Solving a Complex Equation in MathCad

Trying to solve this equation in mathcad prime, i just cant get the form right, i keep getting an error when I know it should be a simple operation.

It has values S (total distance traveled) and X (horizontal distance traveled) known, trying to solve for the max value of H (height) of an object in parabola projection.

I keep setting the variables, guess for the H variable, and then try to solve the equation using Find(H) or Maximize(H) doesnt work. I'll attach an image of the problem. Any help would be greatly appreciated!

Thanks!