input: Write a function to solve a linear equation system of the form: ax + by = c and return the values of x and y. The function should take three arguments a, b, and c and return a tuple (x, y). If the given system of equations is invalid (division by zero), return None.
  def solve_linear_equation_system(a, b, c):
   if a == 0 or b == 0:
   return 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(1, -1, 0) == (0.0, 0.0)
  assert solve_linear_equation_system(0, 1, 1) == None
  assert solve_linear_equation_system(1, 0, 1) == None
  assert solve_linear_equation_system(3, 3, 9) == (3.0, 0.0)
output: def solve_linear_equation_system(a, b, c):
      if a == 0 or b == 0:
          return None
      x = (c - b) / a
      y = (c - a * x) / b
      return x, y