input: 
  def linear_eq(a, b, c):
      """Calculate the objective function of a linear equation in the form of ax + by = c.
      >>> linear_eq(3, 5, 7)
      -7
      """
      x = 0
      y = 0
      return a*x + b*y - c
  
output:     while a*x + b*y != c:
          if a*x + b*y < c:
              x += 1
          else:
              y += 1
      return a*x + b*y - c