Question 69772
The perimeter of a triangle is equal to the sum of all of its sides lengths.

Therefore, in your triangle's case, 

{{{p=3x+7+4x-9+5x+6}}}

Combine like terms!

{{{p=(3x+4x+5x)+(7-9+6)}}}

Simplify.

{{{p=12x+4}}}

And you're done. =)