input: Write a function to solve a linear equation system of the form: ax + by = c. The function should take three arguments a, b, and c and return the values of x and y as a tuple. If the given system is not solvable, return None.
  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, 1, 3)==None
  assert solve_linear_equation_system(0, 0, 0)==None
  assert solve_linear_equation_system(1, 0, 5)==(5.0, None)
output: def solve_linear_equation_system(a, b, c):
      if a == 0 and b == 0:
          return None
      if a == 0:
          return None
      if b == 0:
          return (c / a, None)
      x = c / a
      y = (c - a * x) / b
      return (x, y)