input: def objective_function(a: float, b: float, c: float) -> float:
      """ Calculate the objective function of a linear equation of the form ax + by = c.
      The objective function is defined as the value of the linear combination minus the constant term.
  
      >>> objective_function(3, 5, 7) == 0
      True
      >>> objective_function(1, 2, 3) == 0
      True
      """
output:     return a + b - c