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 I created two variables l and v and want to create l2 variable from LengthContractionRelativistic formula l=Quantity[1,"Meters"] v=Quantity[0.5, "SpeedOfLight"] l2=FormulaData["LengthContractionRelativistic", {"v" ->Subscript["l",0]}] It create l2 like next l==0.866025m but I want create l2 like 0.866025m to use it in expression l3=l2+Quantity[2,"Meters"] and so on.How to assign l2 = 0.866025m by using LengthContractionRelativistic formula?
 You're left with an equation, albeit a very simple one, use Solve to solve the equation, substitute the solution, and set it to l : l2 = l /. First[Solve[l2, l]] 
 Your answer is not correct. I find next topic https://reference.wolfram.com/language/guide/FormulaManipulation.html where in section Extracting Parts of Formulas is link to https://reference.wolfram.com/language/ref/Part.html So correct answer is l2=FormulaData["LengthContractionRelativistic", {"v"->v,Subscript["l",0]->l}] [[2]]