## Algebra 1: Common Core (15th Edition)

The length of the fourth side is $8a-2$.
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. $=39a-31a\qquad-7+5$ $=8a-2$