How many students were there in University of Michigan in 2011?
in 2011 refers to year 2011; in University of Michigan refers to university_name = 'University of Michigan';
SELECT COUNT(*) FROM university AS T1 INNER JOIN university_year AS T2 ON T1.id = T2.university_id WHERE T1.university_name = 'University of Michigan' AND T2.year = 2011