input: Write a function to solve a linear equation system of the form: ax + by = c. Given the input values a, b, and c, the function should return the values of x and y. If the given values are invalid (e.g., division by zero), return None.
  
  assert solve_linear_equation_system(2, 1, 4) == (2.0, 0.0)
  assert solve_linear_equation_system(1, 1, 2) == (2.0, 0.0)
  assert solve_linear_equation_system(0, 1, 3) == None
  assert solve_linear_equation_system(1, 0, 2) == None
  assert solve_linear_equation_system(3, 4, 11) == (3.0, 2.0)
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
          else:
              return None
      if b == 0:
          return c / a, 0
      return c / a, 0