Because of the little square in corner of one angle in the triangle, you know that angle is 90º and that means this is a right triangle.
Becuase it's a right triangle, you can use the Pythagorean Theorem.
"The sum of the squares of the legs equals the square of the hypotenuse."
This is usually stated as
. The key is that
is the side length opposite the 90º angle.
OK, so in your case, you have
![x^2+8^2 = 12^2](https://tex.z-dn.net/?f=x%5E2%2B8%5E2%20%3D%2012%5E2)
Solving that would look like this:
![\begin{aligned}x^2+8^2 &= 12^2\\[0.5em]x^2+64 &= 144\\[0.5em]x^2 &= 80\end{aligned}](https://tex.z-dn.net/?f=%5Cbegin%7Baligned%7Dx%5E2%2B8%5E2%20%26%3D%2012%5E2%5C%5C%5B0.5em%5Dx%5E2%2B64%20%26%3D%20144%5C%5C%5B0.5em%5Dx%5E2%20%26%3D%2080%5Cend%7Baligned%7D)
Then you square root each side
(Techincally it's
, but a negative value doesn't make sense here.0
Throw that into a calculator and you get x ≈ 8.94.