input: Write a function to calculate the objective function of a linear equation given by ax + by = c, where a, b, and c are provided as input parameters. If x and y are not provided, return the string 'x, y not provided'.
  
  def linear_eq(a, b, c, x=None, y=None):
      if x is None or y is None:
          return 'x, y not provided'
      return a*x + b*y - c
  
  assert linear_eq(3, 5, 7, 2, 1) == 0
  assert linear_eq(3, 5, 7, 1, 2) == 0
  assert linear_eq(3, 5, 7, -1, 2) == 5
  assert linear_eq(3, 5, 7) == 'x, y not provided'
  assert linear_eq(3, 5, 7, x=2) == 'x, y not provided'
  assert linear_eq(3, 5, 7, y=1) == 'x, y not provided'
output: def linear_eq(a, b, c, x=None, y=None):
      if x is None or y is None:
          return 'x, y not provided'
      return a*x + b*y - c