Consider the following premises. u_4 -> q_6. q_6. q_6 -> u_4. Can we infer u_4 from them? If possible, name the inference rules being used at each step.
 From u_4 -> q_6, q_6 -> u_4 we can infer u_4 <-> q_6 via biconditional introduction. From u_4 <-> q_6 we can infer q_6 -> u_4 via biconditional elimination. Finally, from q_6 -> u_4, q_6 we can infer u_4 via modus ponens. Therefore, the answer is yes.