Maybe something like this:

questionP21 = 
 HoldForm[((v) ((cc)*2*r5bis/((r3) + (r4))))*((1/
       Sqrt[(r3) + (r4)]))] /. {v -> valke, cc -> convertC, 
   r5bis -> r5}

That's just because Gianluca didn't add replacement rules for r3 and r4. You could easily add those.

You might actually get more flexibility with a template:

equationTemplate = 
 TemplateExpression[((TemplateSlot[
        "valke"]) ((TemplateSlot["convertC"])*2*
        TemplateSlot[
          "r5"]/((TemplateSlot["r3"]) + (TemplateSlot["r4"]))))*((1/
       Sqrt[(TemplateSlot["r3"]) + (TemplateSlot["r4"])])) == 
   TemplateSlot["answer"]]

Then you apply the template:

TemplateApply[equationTemplate, <|"valke" -> valke, "r3" -> r3, 
  "r4" -> r4, "r5" -> r5, "convertC" -> convertC, 
  "answer" -> answerP21|>, InsertionFunction -> HoldForm]

With this approach, you decouple your values from your format. For example, if you don't like the look of 0.25 m^2 + 0.25 m^2 and would like those to automatically combine, you could change your template thus:

equationTemplate = 
 TemplateExpression[((TemplateSlot[
        "valke"]) ((TemplateSlot["convertC"])*2*
        TemplateSlot["r5"]/(TemplateSlot["r3+r4"])))*((1/
       Sqrt[TemplateSlot["r3+r4"]])) == TemplateSlot["answer"]]

and then add another rule in your application:

TemplateApply[equationTemplate, <|"valke" -> valke, "r3" -> r3, 
  "r4" -> r4, "r5" -> r5, "convertC" -> convertC, 
  "answer" -> answerP21, "r3+r4" -> r3 + r4|>, 
 InsertionFunction -> HoldForm]