help plz

Since AC is the diagonal of square ABCD we know that it is a right triangle.

There are 2 methods.

1st Method(45-45-90 Triangle)

Since AC is a diagonal of square ABCD. We know that triangle ABC is a 45-45-90 triangle.
Using our knowledge that it is a 45-45-90 triangle and the ratios will go 1:1:$x = \sqrt{2}$. We have x:x:2 = 1:1:$\sqrt{2}$ 
Dividing 2 by $\sqrt{2}$ we get $2/\sqrt{2}$ which rationalizing the denominator equals $x = \sqrt{2}$.

$x = \sqrt{2}$

Also rationalizing the demoninator involves multiplying a fraction with an irrational denominator with itself/itself. For example rationalizing $2/\sqrt{2}$ in this case would involve multiplying the fraction by $\sqrt{2}/\sqrt{2} = 1$

Another method is easier for beginners(Pythagorean Theorem)

Knowing that the figure above is a square we know BC is also equal to x. Using the pythagorean theorem we get $x^2 + x^2 = 2^2 or 2x^2 = 4$ . Dividing by 2 on both sides we get $x^2 = 2$. Squarerooting on both sides we get $x = +- \sqrt{2}$ However, since a length cannot have negative length.

$x = \sqrt{2}$