Answer
$$13.5 \ cm .$$
Work Step by Step
Since we have a right triangle with legs a, b and the hypotenuse c, then by the Pythagorean theorem, we have $$c^2=a^2+b^2.$$
Since $c=18.5 cm$ and $b= 12.6 cm$, then $a$ is given by
$$ a=\sqrt{c^2-b^2}=\sqrt{18.5^2-12.6^2}=13.5 \ cm .$$
