use the result of the quadratic function

In case you are interested in the positive solution only you could use the “assume, x>0” modifier and assign the result a variable (and you may use “x” as variable name or anything else):

But you don’t need to use Primes symbolics. Alternatively you may use the “root” function or a solve block with “find”.

You may use the root function by providing a meaningful interval for the solution

or by providing a guess value (a negative guess will give you the result -1)

You cannot add an additional constraint if you use the “root” function, but you can if you use a solve block:

Note that this examples were done in legacy Mathcad 15 and a solve block looks quite different in Prime (“Given” is not needed and you have to insert the solve block like a separate component (Menu: Math - Solve block, or Ctrl+7)