You have to know that the symbolics does not understand anything about units. It does not know that 1cm=10mm or the like. So it treats each unit as an unknown variable and therefore can't simplify them.
Checking "Units/Constants in symbolics" is a good idea and you should do it. The symbolics still don't know anything about units, but at least Prime labels these units in the symbolic result as being a unit (you can see that kg, mm, etc. is now shown in bold and blue) and therefore the native numeric engine in Prime can numerically evaluate the variable and of course it simplifies the units as well:
I would suggest other approaches:
1) You may turn the symbolic solve result into a function of the involved variables. That way you could easily compare evaluation with different input values side by side.
2) If you are only interested in a numerical result, you may consider to avoid the symbolics altogether a use one of Primes numeric methods for solving an equation. You have the choice between a solve block with "find" or using the "root" function. The root function could be used in two flavors - either by providing a guess value or by providing an appropriate lower and upper limit for the variable you want to solve for.
Here is the last mentioned method - root-function and providing a range from 0 gm cm^2 to 500 gm cm^2.
I had to get rid of the derivative of theta(t) which cancels anyway and also had to get all expressions to one side of the equation because "root" will not solve equations but rather looks for the zero of a function:
Prime 9 worksheet attached
PTC Mathcad prime 9.0.0.0