Question 27471
If the sides of your triangle were 3,4, and 5, you would find the perimeter (the distance around the outside) by adding 3+4+5=12.  You do the same for this problem.

  3x  + 7
  4x  - 9
+ 5x  + 6
-------------
 12x  + 4

(Since all the x's are positive, you just add them together to get 12x and then since the 9 is negative, you add the 6 and 7, and then subtract 9 to get 4)