<PRE><B>Question 93839</B>
Start with the given formula for finding the perimeter of a triangle


{{{P=S[1]+S[2]+S[3]}}} where P is the perimeter and the S terms are the side lengths 



{{{P=(3x +7)+( 4x -9)+ (5x +6)}}} Plug in the given sides


{{{P=3x+7+4x-9+5x+6}}}   Remove the parenthesis


{{{P=(3x+4x+5x)+(7-9+6)}}} Group like terms


{{{P=12x+4}}} Combine like terms



So the expression that represents the perimeter is {{{P=12x+4}}}

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