input: 
  def objective_function(a, b, c):
      """
      Calculate the objective function of a linear equation ax + by = c, returning the value of the left-hand side without the constant 'c'.
      >>> objective_function(3, 5, 7)
      3*x + 5*y
      >>> objective_function(2, 4, 6)
      2*x + 4*y
      """
      return f"{a}*x + {b}*y"
  
output:     return f"{a}*x + {b}*y"