input: 
  def linear_eq(a: float, b: float, c: float) -> float:
      """This function calculates the objective function of a linear equation of the form ax + by = c.
      It returns the expression a*x + b*y - c.
      
      >>> linear_eq(3, 5, 7)
      a*x + b*y - 7
      """
  
output:     return f"a*x + b*y - {c}"