The variable (2 element vector) S is symbolically defined, but numerically undefined, because next to the unit cm, it contains a variable cm that is undefined.
Now you could define this cm as being 1 cm, but I think you should approach the matter in a different way.
Using 'solve,x' means that you are using the symbolic processor. You'd normally do that only if you are interested in the symbolic solution. And for your particular problem there are two solutions, due to the fact it's a quadratic expression in x.
Without using a solve block, you can use the function root, which needs a guess value:

or two additional parameters to define the range in which x is to be found:

The other method does use the symbolics, but with all parameters numerically undefined:

Note that while Prime allows magic numbers and units in symbolics, it makes a mess of the result too often, as in your case where it changed the unit cm to a variable.
In this case I would advise to use a variable name for 0.0030 and 0.85. A symbolic expression should ideally only contain variables and constants (1, 2, or e, pi), but never any 'magic numbers' (Unnamed numerical constants as described here) such as your 0.0030 and 0.85.
It might look like:

Success!
Luc