The first thing we can take from this is that to find the sum of all the angles in a polygon (and a triangle is a polygon), we subtract the number of sides by 2 and multiply that by 180. Since there are 3 sides in a triangle, we can subtract that by 2 to get 3-2=1. Next, we can multiply that by 180, and since anything multiplied by 1 is itself, we know that the angles in a triangle add up to 180.
Next, what I personally like to do is say that if things are in a ratio, there’s always the base value of 1. I want to find what that 1 in the ratio is (as 2+4+9=15, not 180, 1 is not 1), and I can represent that by a value x. After that, I know that x multiplied by 2 = the angle that has the ratio of 2, x*4= the angle that has the ratio of 4, and x*9=the angle that has the ratio of 9. Next, since the angles all add up to 180, I know that x*2+x*4+x*9=180. We then can combine like terms to get x(2+4+9)=x*15=180. Dividing both sides by 15 to solve for x, we get x=12. Therefore, the measures of the angles are 12*2=24, 12*4=48, and 12*9=108.
Feel free to ask further questions!