input: Write a python function to determine the highest value of k for which k % x equals y.
  assert get_max_k(15, 10, 5) == 15
  assert get_max_k(187, 10, 5) == 185
  assert get_max_k(16, 11, 1) == 12
output: def get_max_k(n, x, y):
      for k in range(n, y, -1):
          if k % x == y:
              return k
      return None