salut
1) 3x-21>=0 => 3x>=21 => x>=7 S=[ 7 ; + inf [
2) 2x-7<=0 => 2x<7 => x< 7/2 S= ] - inf ; 7/2 ]
3)2x+9>2 => 2x>-7 => x > -7/2 S= ] -7/2 ; + inf [
4) -3x+2>=6 => -3x>=4 => x<= -4/3 S=] -inf ; -4/3 ]
5) 5x+13<8x-2 => -3x<-15 => x>5 S= ] 5 ; +inf [
6) -2x+4>=3x-5 => -5x>=9 => x<=9/5 S=] - inf ; 9/5 ]