input: 
  def linear_eq(a: int, b: int, c: int) -> int:
      """ Calculate the objective function of a linear equation ax + by = c.
      The function should return the value of a*x + b*y - c.
      >>> linear_eq(3, 5, 7)
      -7
      """
  
output:     x = 0
      y = 0
      return a * x + b * y - c