input: Write a function to solve a linear equation system of the form: ax + by = c. The function should return the values of x and y as a tuple. If the given coefficients are invalid (e.g., division by zero), the function should return None.
  
  def solve_linear_equation_system(a, b, c):
      if a == 0 and b == 0:
          return None
      if a == 0:
          y = c / b
          return None, y
      if b == 0:
          x = c / a
          return x, None
      x = (c - b) / a
      y = (c - a*x) / b
      return x, y
  
  assert solve_linear_equation_system(2, 1, 4) == (2.0, 2.0)
  assert solve_linear_equation_system(0, 5, 10) == (None, 2.0)
  assert solve_linear_equation_system(3, 0, 9) == (3.0, None)
  assert solve_linear_equation_system(0, 0, 0) == None
  assert solve_linear_equation_system(1, 1, 2) == (1.0, 1.0)
output: def solve_linear_equation_system(a, b, c):
      if a == 0 and b == 0:
          return None
      if a == 0:
          if b == 0:
              return None
          y = c / b
          return None, y
      if b == 0:
          x = c / a
          return x, None
      x = c / a
      y = (c - a * x) / b
      return x, y