Following each step accordingly:
The simplicity relies on the fact, you must put those with a variable on one side and without a variable on the other side. I moved the 6 to the other side since -2a and -a were already there , so the -6 changed to +6.
Then I simply solved it by multiplying, adding and dividing.
-2(a+3)-a=0
-2a-6-a=0
-2a-a=0+6
-3a=6
a=6/-3
a=-2
Best of wishes to you :)
Happy Wednesday tomorrow!