This problem seems trivial at first, but turns out to be very complicated. Most people trying to solve this start out with thinking that you can just divide by 7 and multiply by 5 and accounting for the remainder. But then you realize you need to account for national holidays, in al countries that your app is used, for all future dates, and potentially for dates in the past.
My advice, choose one of these options:
Which option fits you best will depend on how often you need to do the calculation, how much time you want to spend maintaining a list of public holidays and how comfortable you are with coding things in a language like Java.