21-3а+9=4а-5
-3а-4а=-5-9-21
-7а=-35
а=5

21-3(a-3)=4a-5      

21-3a+9=4a-5 

-3a-4a= -21-9-5 

-7a = -35 

a= (-7): (-35)

a= 5

 0.1(x-2)-0.4(x+3)=0.4-0.6x

 0,1x-0.2-0.4x-1.2=0.4-0.6x 

 0.1x-0.4x+0.6x=0.4+0.2+1.2

 0.3x=1.8 

 x=1.8:0.3 

 x=6