An 86 yr old male complains of cough and blood in his sputum for the past two days. On admission, his temperature is 103deg F. physical examination reveals rales in his right lung, and x- ray examination shows increased density in the right middle lobe. A sputum smear shows many Gram positive coci, confirmed by sputum culture as penicillinase producing staphylococcus aureus. Which of the following agents should be given?