Reduce solving solution (Intermediate calculations)

I'm no expert in symbolics, but in this case you don't need that. You can use a solve block.  (Also - be aware float actually drops accuracy)  

The symbolic solver does not knwo about units. It treats your 10 mm and 8 mm each as a constant times a variable 'mm' and  does NOT relate this variable to the unit mm.

First solve your symbolic expression, then fill in the values:

(The solution runs off into the right...)

It's up to you to decide which of the two possible solutions to this quadratic expression is the right one. That may(...) be accomplished with an assume statement.

Success!
Luc