input: def solve_linear_equation_system(a, b, c):
      """Solve a linear equation system of the form: ax + by = c.
      >>> x, y = solve_linear_equation_system(2, 1, 4)
      >>> print("x = {}, y = {}".format(x, y))
      x = 2.0, y = 2.0
      """
      x = (c - b) / a
      y = (c - a*x) / b
      return x, y
output:     x = (c - b) / a
      y = (c - a*x) / b
      return x, y