x+21=36````````

5/12x-1/3=-1/4
解: 方程两边同时加 1/3
5/12x -1/3 + 1/3 = -1/4 + 1/3
5/12x = - 1/12
方程两边同时 乘以 12/5
12/5 * 5/12 x = -1/12 * 12/5
x= -1/5

1/9 = x/3 -1/6
解 方程两边同时加1/6
1/9 + 1/6 = x/3 - 1/6 + 1/6
5/ 18 = x/3
方程两边同时乘以3
5/18 * 3 = x/3 * 3
x = 5/6

楼主在问什么?