input: Write a function to explain the approach for solving a linear programming problem using the Simplex method in Python. If the function receives a valid linear programming problem, it should explain how the Simplex Method is applied to solve it. If the input problem is not valid, the function should return an error message indicating that the input is not a linear programming problem.
  assert simplex_explanation("max 3x1 + 2x2 subject to 2x1 + x2 <= 18, 2x1 + 3x2 <= 42, 3x1 + x2 <= 24, x1, x2 >= 0")
  assert simplex_explanation("min 2x + 3y subject to x + y >= 2, x, y >= 0")
  assert simplex_explanation("solve this") == "Error: Input is not a valid linear programming problem."
  assert simplex_explanation("") == "Error: Input is not a valid linear programming problem."
  assert simplex_explanation("max x1 + x2") == "Error: Input is not a valid linear programming problem."
output: def simplex_explanation(problem):
      if "max" in problem or "min" in problem and "subject to" in problem:
          return "The Simplex Method is applied by converting the linear programming problem into its standard form, introducing slack variables for inequality constraints, and iteratively improving the solution by moving from one vertex of the feasible region to another until the optimal solution is found."
      else:
          return "Error: Input is not a valid linear programming problem."