input: def solve_linear_equation_system(a, b, c):
      """Solve a linear equation system of the form ax + by = c.
      >>> solve_linear_equation_system(2, 1, 4)
      (2.0, 2.0)
      >>> solve_linear_equation_system(1, 1, 2)
      (2.0, 0.0)
      """
      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