well first you add like terms 21=6-1x-4x 21=6-5x now, move your x value in front of the = 21=6-5x -5x -5x 21-5x=6 next, move the 21 to behind the = 21-5x=6 -21 -21 -5x=6-21 then do 6-21, which equals -15, so -5x=-15 next, divide both sides by -5 -5x=-15 -5 -5 x=3