input: def solve_linear_equation_system(a: float, b: float, c: float) -> tuple:
      """Solve a linear equation system of the form: ax + by = c.
      Return the values of x and y.
      >>> solve_linear_equation_system(2, 1, 4)
      (2.0, 2.0)
      """
      x = (c - b) / a
      y = (c - a * x) / b
      return x, y
output:     if a == 0 and b == 0:
          raise ValueError("The equation system is not solvable.")
      elif a == 0:
          y = c / b
          return None, y
      elif b == 0:
          x = c / a
          return x, None
      else:
          x = (c - b) / a
          y = (c - a * x) / b
          return x, y