A few doubts

Question

•Why do we have to use 'pero' in the first sentence?•In the hint provided, it was given that tardar tiempo = take long, then in the answer key, why is 'mucho' used?

Answer

Hola Anshu
About "pero", this is how this sort of sentence is expressed in a more natural way in Spanish, using "pero" at the beginning -because in English you don't need that, we have given the "literary" note so that you realize you need to include "but" in your Spanish sentence.
About adding "mucho" to the sentence for "tardar mucho tiempo", that word was missing from the hint, so it is now saying "to take long = tardar mucho tiempo" 
Un saludo
Inma