There were [tex]N[/tex] balls but we took [tex]A[/tex] balls out, so there are now [tex]N-A[/tex] balls. We add [tex]B[/tex] balls and now we have [tex]N-A+B[/tex] balls.
The program that computes this (I will use python as language has not been specified is the following):
n, a, b = int(input()), int(input()), int(input())
print(f"There are {n-a+b} balls in the box")
# Hope this helps