input: 
  from typing import List
  
  def linear_eq(a: float, b: float, c: float) -> float:
      """
      Calculate the objective function of a linear equation ax + by = c.
  
      >>> linear_eq(3, 5, 7)
      0
      >>> linear_eq(2, 4, 6)
      -2
      """
      return a*x + b*y - c
  
output:     x = 1
      y = 1
      while a*x + b*y != c:
          if a*x + b*y > c:
              y -= 1
          else:
              x += 1
      return a*x + b*y - c