You're trying to symbolically solve this. NOTE that the symbolic processor of Mathcad (and Prime) does NOT know about units. It treats your units m, s and kg as unknown variables, and also the gravitation g. That's why it produces answers in terms of root of kg and root of g.
One way of obtaining a simple answer is to use the root function:
Not that it does not give you units in the result, even though it's the result of a pure numeric approximation.
If you want to work symbolically, then FIRST solve your symbolic problem, then if necessary select the symbolic solution you prefer, then provide the data:
Apart from using the symbolic "solve" in conjunction with "assume, ALL>0" and the "root command you could also use a solve block.
Note that root and solve block are numeric methods which is the reason that you have to assign m a value (or label m as unit) - otherwise the numerics will throw an error.
