If date are fixed or you are going to pass these someway to the query then you can ctry this-
SELECT T1.id_user, T1.equity-T2.equity AS summary
FROM YourTable T1
LEFT JOIN YourTable T2 ON T1.id_user=T2.id_user
WHERE T1.schedule='2015-09-29' AND T2.schedule='2015-09-28'
If you want it to make dynamic, please let me know.
Hope, it helps :)