What was the surface played on when they were a runner-up in 2007?
CREATE TABLE "mixed_doubles_11_5_6" (
  "outcome" text,
  "year" real,
  "championship" text,
  "surface" text,
  "partner" text,
  "opponents_in_the_final" text,
  "score_in_the_final" text
);
SELECT "surface" FROM "mixed_doubles_11_5_6" WHERE "outcome"='runner-up' AND "year"=2007;