2/5x-5/9x=21

2/5x-5/9x=21
2/5x-5/9x=21

2/5x-5/9x=21
2/5x-5/9x=21
(2/5-5/9)x=21
(18-25)/45 x=21
-7/45 x =21
所以x=21/(-7/45)=-135

(2/5-5/9)x=21
-7/45x=21
x=-135

因为2/5x-5/9x=21
合并同类项，得
(2/5-5/9)x=21
即 (18-25)/45 x=21
整理得，-7/45 x =21
解得x=-135
上述是完整版过程

先通分得18/45X-25/45X=21再得-7/45=21得X=1/135