The answer says SUM. You can only ADD like terms or things that end the exact same. You can't add 11x^2 to x. So the only thing you can put together in this problem is -5 and +4. Everything else stays the same in the problem but it all has to be in the answer. Don't think of it as adding, think of it as combining. Make sure you watch the signs in front of every term when combining them.
because the interior angles of a triangle = 180° so if you subtract 90° you get 90. in order to get 90 you can use the equation x + 2x = 90 which you will then get x = 30
