Answer
The unknown length is $5$ meters.
Work Step by Step
Let $c$ = length of the hypotenuse
Use the Pythagorean theorem's formula $c^2=a^2+b^2$ where c= length of the hypotenuse, and a and b are the lengths of the legs of the righ triangle to have:
$\\c^2=3^2+4^2
\\c^2=9+16
\\c^2=25
\\c = \sqrt{25}
\\c=5$
