Answer
$17.3 \ m$
Work Step by Step
Since we have a right triangle with legs $a$, $b$ and a hypotenuse $c$, then by the Pythagorean theorem, we have $$c^2=a^2+b^2.$$ Since $c=25\ m $ and $a=18\ m$, then $b$ is given by
$$ b=\sqrt{c^2-a^2}=\sqrt{25^2-18^2}=17.349 \ m\approx17.3 \ m .$$
