6 - a = 

 (9a - 18)
First, simplify 

 to 

 / Your problem should look like: 
 Second,
Second, multiply both sides by 3. / Your problem should look like: 
 Third,
Third, expand. / Your problem should look like: 
 Fourth,
Fourth, add 3a to both sides. / Your problem should look like: 
 Fifth,
Fifth, simplify 9a - 18 + 3a to 12a - 18. / Your problem should look like: 
 Sixth,
Sixth, add 18 to both sides. / Your problem should look like: 
 Seventh,
Seventh, add 18 + 18 to get 36. / Your problem should look like: 
 Eighth,
Eighth, divide both sides by 12. / Your problem should look like: 
 Ninth,
Ninth, since 3 goes into 12 to get 36, simplify 

 to 3. / Your problem should look like: 
 Tenth,
Tenth, switch sides. / Your problem should look like: 

Answer: 
a = 3 (one solution)