You can solve this problem by using similar triangles. You can tell that the two triangles are similar by AAA (since two angles are the same, the third must be too).
Now that we've established that the triangles are similar, you need to identify the corresponding sides:
12 corresponds to 28
9 corresponds to x
In order to find x, you'll need to find the scale factor, which you can find by dividing 28 by 12:
28 ÷ 12 = 7/3
Now that you know that 7/3 is the scale factor, you can multiply it by 9 to find x:
