input: Write a function to calculate the objective function of a linear equation of the form ax + by = c. If the given coefficients are not provided, return None.
  def linear_eq(a=None, b=None, c=None):
      if a is None or b is None or c is None:
          return None
      return a*x + b*y - c
  
  assert linear_eq(3, 5, 7) == 3*x + 5*y - 7
  assert linear_eq() == None
  assert linear_eq(a=3) == None
  assert linear_eq(b=5) == None
  assert linear_eq(c=7) == None
output: def linear_eq(a=None, b=None, c=None):
      if a is None or b is None or c is None:
          return None
      return a*x + b*y - c