Answer
The length of the fourth side is $8a-2$.
Work Step by Step
The sum of the lengths of the given three sides is
$ 9a+5a+1+17a-6=\qquad$ .....gather like terms.
$=9a+5a+17a\qquad+1-6$
$=31a-5$
To find the fourth side, we subtract the sum of the given sides from the perimeter.
$(39a-7)-(31a-5)=\qquad.$...remove parentheses.
$=39a-7-31a+5=\qquad$ ...gather like terms.
$=39a-31a\qquad-7+5$
$=8a-2$
