Math equation solve issue

Hi experts,

I am trying to use Mathcad to solve the equation based on Equations 4 and equation 6.

below equations are from this paper.

https://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=9913417

when I try to use that to solve the equation. Mathcad can't find the answer in equation 6. but it is OK to get the answer by using eqaution4.

I know equations 4 and 6 are different, but I don't understand why I can find it. but this paper can.

so I would like to know if my operation is wrong.

ps: the version of Mathcad is Mathcad 15