Mathcad's symbolics doesn't know anything about units and treats them as unknown variables. m and mm are two different variables as seem by the symbolics and so they are not cancelled/simplified.
Solution is to assign the symbolic calculation to a variable and let this variable evaluate numerically.
Or you can also use an inline evalution if you just want to check the variabe kn and do not intend to use it later.
If you only want a numeric solution, you could also do without any symbolics and use a solve block
or the root() function
If, on the other hand, you used the symbolic solve on purpose and you had expected a general symbolic result, you would have to clear all variables in use for symbolics evaluation. Your posting is missing the information what kind of result you have expected, you simply wrote that you didn't like what you got:
or follow it with a numeric inline evaluation
