Length

Half of a rectangle = Right triangle (Because angles of a rectangle is $90°$ , same as half of a rectangle which is triangle). In this case, the diagonal of the rectangle is the hypotenuse of the right triangle, so we can use Pythagorean Theorem:

$a^{2} + b^{2} = c^{2}$

$3^{2} + 4^{2} = ?^{2}$

$25 = ?^2$

$? = \sqrt{25} = 5$

So, the length of the third side is 5.

In any right triangle, the square of the hypotenuse is equal to the sum of the squares of the two legs.

Just use the Pythagorean theorem

It is a $3-4-5$ right triangle. One type of special right triangle.

by Pythagoras theorem hypotenuse^2 = 3^2 + 4^2 when we solve we get 5 as answer.