WASHINGTON (AP) — Long-term U.S. mortgage rates slid to new lows for the year this week amid market upheaval stoked by Britain's vote to leave the European Union. Rates are at three-year lows at the height of the spring home buying season. Mortgage buyer Freddie Mac says the average 30-year fixed-rate mortgage fell to 3.48 percent from 3.56 percent last week. The benchmark rate is down sharply from 4.08 percent a year ago. The average for the 15-year fixed-rate mortgage declined to 2.78 percent from 2.83 percent last week.