Let's go:
Comparing the triangle 1 with the biggest triangle:
[tex]\frac{4}{x} = \frac{x}{13} \\\\ x^2 = 4.13 \\\ x^2 = 52 \\\ x = \sqrt{52} \\\ x = 2\sqrt{13}[/tex]
The number in green box is 13.
I hope I helped you.
