input: 
  def linear_eq(a: float, b: float, c: float) -> float:
      """
      Calculate the objective function for a given linear equation of the form ax + by = c.
      Returns the value of the objective function after substituting in the values of x and y.
      >>> linear_eq(3, 5, 7)
      -7.0
      """
  
output:     x = -1
      y = 2
      return a * x + b * y - c