Answer
The unknown length of the triangle is $\approx 5.8$ mm.
Work Step by Step
Let $x =$ unknown length of the triangle
$a^{2} + b^{2} = c^{2}$
$x^{2} + 9.1^{2} = 10.8^{2}$
$x^{2} + 82.81 = 116.64$
$x^{2} = 33.83$
$x = \sqrt 33.83$
$x = 5.816...$
$x \approx 5.8$ mm