I'm guessing that you know about 45-45-90 and 30-60-90 triangles.
In 45-45-90 triangles, the legs are in a ratio such that the hypotenuse is [tex]\sqrt{2}[/tex] times the legs.
In 30-60-90 triangles, the legs are in a ratio of 1:[tex]\sqrt{3}[/tex]:2
In the picture given we have been given a value of the side of the 45-45-90 triangle. We can deduce that the value of the hypotenuse is 9[tex]\sqrt{2}[/tex] because of the ratio of the sides of a 45-45-90 triangle.
The 30-60-90 triangle shares the same hypotenuse, and so we now know a value of one of its sides as well. The ratio of the side of value 'x' and the hypotenuse is [tex]\sqrt{3}[/tex] : 2
We can create an equation to solve for x using these ratios:
[tex]\frac{\sqrt{3} }{2}[/tex] = [tex]\frac{x}{9\sqrt{2} }[/tex]
Cross multiply:
2x = 9[tex]\sqrt{6}[/tex]
x = [tex]\frac{9\sqrt{6} }{2}[/tex]
Option C