30-3（3-X）=4X
30-9+3x=4x
21=x
x=21
3X+12*6=15X+12
72-12=15x-3x
12x=60
x=5
4X-2(50-X)=20
4x-100+2x=20
6x=120
x=20
12(X-13)=10(X+3)
12x-156=10x+30
2x=186
x=93
100-X=3(20+X)
100-x=60+3x
40=4x
x=100.5（X-1.2）=1.5X=1