So the total line equals 180 degrees, and the that a little square is 90 degrees. And 180 - 90 = 90. So now we now that both sides are 90 degrees. And for the right side we know that half of it is 39 degrees, and we need to find the other or x. So we do 90 - 39 which is 51, and now we have to figure out what multiplied by 3 gives 51. So it's like 3 × _ = 51. And that blank is 17, and you get that when you divide 51 by 3. X = 17
