3 - 6/7x = 21 first we must get x by itself, to this we subtract 3 from both sides
3-3-6/7x= 21-3 the 3's on the left side cancel out
-6/7x = 18 next multiply both sides by 7
7(-6/7)=18(7) the 7's cancel out because when you cross multiply, 7 goes into 7 once.
-6x = 126 now divide each side by -6, since it is being multiplied by x you divide to undo that.
-6/-6x = 126/-6 -6/-6 is just 1
1x = -21
x =- 21