input: Write a function to solve a linear equation system of the form: ax + by = c. The function should take three parameters: a, b, and c, and return the values of x and y as a tuple. If the given coefficients do not allow solving the equation, such as when both a and b are zero, the function should return None.
  
  def solve_linear_equation_system(a, b, c):
      if a == 0 and b == 0:
          return None
      x = (c - b) / a if a != 0 else None
      y = (c - a*x) / b if b != 0 else None
      return (x, y)
  
  assert solve_linear_equation_system(2, 1, 4) == (2.0, 2.0)
  assert solve_linear_equation_system(1, 1, 2) == (1.0, 1.0)
  assert solve_linear_equation_system(0, 0, 4) == None
  assert solve_linear_equation_system(0, 1, 3) == None
output: def solve_linear_equation_system(a, b, c):
      if a == 0 and b == 0:
          return None
      x = (c - b) / a if a != 0 else None
      y = (c - a*x) / b if b != 0 else None
      return (x, y)