So, we know that a triangle's sides must add up to have a sum of 180. If you add up your given lengths and you should get 47, not including x. Then if you look at the left side/the one without a given length and it should be identical to the 21, so you know that side is 21. So now add 21 to 47, you should get 68. Finally, you subtract 68 from 180 like this: 180-68, and it should equal 112. So you're answer is 112.
