Answer:
25.5
Explanation:
we're trying to isolate x to find out what it is!
first, the line equals 180 so we'll set that as the base equation
both angles added together should equal 180, so that's how i got this equation to start with
180 = 129 + (2x+9) from here you have to subtract 129 on both sides
51 = 2x + 9 subtract 9 from both sides
51 = 2x next divide both sides by 2
25.5 = x