Treatment for the roots of my back teeth takes time. And you have to go back many times.
It takes time and a lot of trips to the dentist to treat the roots of my molars.
奥歯 = molars, back teeth
根幹 = moots
回数 = frequency, number of times; でも、直訳したら「frequency is also needed」だったら変な英語になります。英語では、何の回数だかを説明しなければなりません(trips to the dentist, go back many timesなど)
アメリカ合衆国 
