As ttokoro already wrote the reason your attempt is failing is Primes auto-label "feature". Its quite often a PITA, especially when you are using symbolic calculations. In the attached file you find the corrected version - manually all occurencies of U.0 are labeled the same - either all "-" (meaning "automatic") or all "variable".
But as you are just looking for a numeric result, I would suggest not using the symbolic engine at all!
Prime provides two numeric means to solve an equation numerically - either use a solve block or use the "root" command. The solve block takes up more space and looks ugly in Prime but its good if you have additional constraints. In your case I would suggest using the "root" command.
Furthermore you should use units throughout - thats one of the strengths of Mathcad!
More details in the attached sheet
