Consider the following premises. There is at least one x_4 for which x_4 is a lawyer if and only if x_4 will take a plane. For all x_4, x_4 is a lawyer if and only if x_4 will take a plane. Can we infer the following from them? If we can, name the inference rule being used: There is at least one x_4 for which x_4 is a lawyer if and only if x_4 will take a plane.
That is one of the premises. Therefore, the answer is yes.