Given equation:

21=4x-9-x

21=3x-9

add 9 to both sides of the equation to isolate the variable term:

21+9=3x-9+9

3x=30

divide both sides by 3

\((3x)/(3)=(30)/(3)\)

x=10

21=4x-9-x

21=3x-9

add 9 to both sides of the equation to isolate the variable term:

21+9=3x-9+9

3x=30

divide both sides by 3

\((3x)/(3)=(30)/(3)\)

x=10