11. Bing Dwen Dwen practiced skiing for a week, during which the average length of skiing per day for the last four days was 4 kilometers more than the average length of skiing per day for the first three days, and the average length of skiing per day for the last three days was 3 kilometers more than the average length of skiing per day for the first four days. The total length of skiing for the last three days was $\qquad$ kilometers more than the total length of skiing for the first three days.
$12$