X- 21/12= 93/18
x= 93/18- 21/12
x= 186/36 -63/36
x= 123/36
---------------------
3/18x - 2015/54 = 691/45
3/18x = 10075/270 + 4146/270
3/18x = 14220/270
x = 316
-----------------------
x-553/21 = 2805/28
x= 58905/588 + 15484/588
x= 58905/588
------------------------
x= 846/25 +1487/35
x=11844/350 +14870/350
x=26714/350