Consider the following premises. For all x_9, x_9 will take a plane. If William were sad, then William is not taking a plane. Can we infer the following from them? If we can, name the inference rule being used: William is not sad.
 From the fact that for all x_9, x_9 will take a plane we can infer that William is taking a plane via universal instantiation. Finally, from the fact that if William were sad, then William is not taking a plane, and that William is taking a plane we can infer that William is not sad via modus tollens. Therefore, the answer is yes.