Getting catagory title total number of questions and answers order by number of questions
SELECT @no := @no + 1 AS no
  ,catagory
  ,number_of_questions
  ,number_of_answers
FROM (
SELECT c.title AS catagory        
    ,COUNT(DISTINCT q.id) AS number_of_questions 
    ,COUNT(a.id) AS number_of_answers
FROM material_univarcity_list_content AS c
LEFT JOIN user_questions AS q ON q.note_id = c.id
LEFT JOIN user_answers AS a ON a.question_id = q.id
GROUP BY c.title ) AS t1
CROSS JOIN (SELECT @no := 0) AS t2
ORDER BY number_of_questions DESC;

Since you use MySQL 5.7 which doesn't support window functions like ROW_NUMBER you need to use user variables to emulate it.