9 9/28- (4 5/21- х)= 6  2/7

9 9/28-4 5/21+х=6 2/7х=6 2/7-9 9/28+4 5/21х=44/7-261/28+89/21х=528/84-783/84+356/84х=101/84х=1 17/84