Move all terms containing a to the left side of the equation.
Subtract 3a from both sides of the equation.
Subtract 3a from 6a.
Move all terms not containing a to the right side of the equation.
Add 9 to both sides of the equation.
Add 12 and 9.
Divide each term by 3 and simplify.
Divide each term in 3a=21 by 3.
Cancel the common factor of 3.
Cancel the common factor.
Divide a by 1.
Divide 21 by 3.
Solve for a 6a-9=12+3a