ANSWER
x = 6.2
EXPLANATION
In similar triangles, the ratio between corresponding sides is constant.
For the right triangle on the right we have the base = x and the hypotenuse = 9
For the big triangle, the one where the right angle is at the top, we have the base = 9 and the hypotenuse = 13:
[tex]\begin{gathered} \frac{b_1}{h_1}=\frac{b_2}{h_2} \\ \frac{x}{9}=\frac{9}{13} \end{gathered}[/tex]
Solving for x:
[tex]x=\frac{9^2}{13}\approx6.2[/tex]
