MCI Communications said it will cut its interstate long-distance telephone rates by 4.8% on Aug. 1, matching the rate reductions proposed last week by AT&T . . . Advanced Micro Devices and Monolithic Memories said they have signed a definitive agreement for the merger of the two companies . . . CFC Acquisition said it withdrew its $469-million offer for Church's Fried Chicken because of Church's refusal to negotiate a reasonable price . . . Union Bancorp of Los Angeles said it has agreed to acquire Western Bank Holding Co. of Bellevue, Wash., for $3.5 million . . . Shareholders of L.F. Rothschild, Unterberg, Towbin Holdings approved changing the firm's name to L.F. Rothschild Holdings . . . The Federal Reserve said the average yield for one-year T-bills, the most popular index for making changes in adjustable-rate mortgages, rose to 7.03% from 6.88% the week before . . . NCR Corp. said the Securities and Exchange Commission is investigating its firing of 13 workers, who reportedly were involved in a trading scheme involving NCR stock options.