Answer:
12
Explanation:
these are 2 similar triangles, as due to the construction we have 3 corresponding pairs of equal angles (the 47° angle is a given as equal, then due to the rules of crossing lines the inner angles as S must be equal as opposite angles, and that automatically makes the third angles equal, as the sum of all angles in a triangle must be always 180°).
in similar triangles the length ratio of corresponding sides is the same for all corresponding sides and lengths in these triangles.
so, we can use the sides opposite of the 47° angles to find that ratio :
21/9 = 7/3
and now also the sides 28 and x must have the same ratio :
28/x = 7/3
4/x = 1/3
12/x = 1
x = 12