The equation Mary Margaret writes on the board is close to accurate, although her notation is not done well. Because of her improper notation, the equation appears to be F times x is equal to the opposite of F times y. The actual equation should be F sub x is equal to the opposite of F sub y. Her request that someone solve for x is nonsense, as x is not its own element, it is just a way of notating the first force F, which is the force acted upon the first body, from the second force.
Her later attempt to demonstrate Newton's third law is even more ridiculous and demonstrates only that she has no understanding of Newton's third law and should probably take an introductory physics class before she tries to teach anything other than building birdhouses.