A 23 year old man involved in a A and is brought to the emergency room with a displaced fracture of the distal third of his left humeral shaft. On his right side he has a displaced fracture of the surgical neck of his humerus as well as a fracture of the medial epicondyle of his distal humerus. He complains of pain in his both arms as well as the inability to move pa of his hand. On physical examination, his arm is swollen with a deformity at the corresponding pas of his humerus. His motor examination is abnormal.Which of the following muscles will this patient most likely have trouble using secondary to the nerve injury sustained in his accident?