Answer
Unknown length of the triangle is $\approx 13.2$ cm
Work Step by Step
Let $x =$ unknown length of the triangle
$a^{2} + b^{2} = c^{2}$
$9^{2} + x^{2} = 16^{2}$
$81 + x^{2} = 256$
$x^{2} = 175$
$x = \sqrt 175$
$x = 13.228...$
$x \approx 13.2$ cm