We can notice that in the two triangles,
1. two angles are equal and
2. they share a common line(side).
This shows that the two triangles are similar.
∴6.3/13.6 = 8.1/x
6.3x = 8.1 × 13.6
= 110.16
x =110.16/6.3
= 17.4857
To the nearest tenth,
Answer = 17.5