CREATE TABLE table_51709 ("Week" FLOAT, "Date" TEXT, "Opponent" TEXT, "Result" TEXT, "Attendance" FLOAT)

In what week was the Result L 15-13?
SELECT AVG("Week") FROM table_51709 WHERE "Result" = 'l 15-13'
SELECT AVG("Week") FROM "table_51709" WHERE "Result" = 'l 15-13'