Accessing another directory

[cenzuratu]

I have a function declared in a header which is located on Project/Calculs on namespace Project::Calculs
I need to use it in a cpp which is locatted on Project/UI on namespace Project::UI

What header should i include?
I guess i need to use ... too

  using namespace Proiect::Calculs; //but it doesn't work 

i need it for school , i can't declare it on the cpp.. it would have been easy