5a - 6 - 2a = 12
First step combine your like terms: 5a - 2a - 6 becomes 3a - 6 = 12
Second step get the variable by itself, so add 6 to both sides of the equal sign, so your new equation is 3a = 18.
Last step: remember get the variable by itself so since it is multiplying do the opposite, so divide both sides by 3, and your answer is 6.