Answer:

Explanation:
Since the smaller triangles have a side of the same length, they share one side, and the included angle is 90°, the triangles are congruent.
This means the length of the side to the left is also x as shown in the image below.
The bigger triangle is also right and has side lengths of x, x, and 6 (the hypotenuse), thus:

Operating:


Since 18=9*2


The options are not clear enough, so I cannot make a choice.