#### Answer

The missing length is $9x-1$.

#### Work Step by Step

The sum of the two given side lengths is
$ 2x+5+5x-1=\quad $.....gather like terms.
$=2x+5x+5-1$
$=7x+4$
To find the third side, we subtract the sum of the given two sides from the perimeter.
$(16x+3)-(7x+4)=\qquad.$...remove parentheses.
$=16x+3-7x-4=\qquad$ .....gather like terms.
$=16x-7x+3-4$
$=9x-1$