Question

Help me plz:) dont get it

Answer 1

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 $a^2+b^2=c^2$.  The key is that $c$ is the side length opposite the 90º angle.

OK, so in your case, you have

   $x^2+8^2 = 12^2$

Solving that would look like this:

    $\begin{aligned}x^2+8^2 &= 12^2\\[0.5em]x^2+64 &= 144\\[0.5em]x^2 &= 80\end{aligned}$

Then you square root each side

    $x = \sqrt{80}$  

    (Techincally it's $x = \pm \sqrt{80}$, but a negative value doesn't make sense here.0

Throw that into a calculator and you get x ≈ 8.94.