The lengths of the three sides of a triangle are $7, x+4$ and $2 x+1$. The perimeter of the triangle is 36 . What is the length of the longest side of the triangle?
(A) 7
(B) 12
(C) 17
(D) 15
(E) 16
Since the perimeter of the triangle is 36 , then $7+(x+4)+(2 x+1)=36$ or $3 x+12=36$ or $3 x=24$ or $x=8$.

Thus, the lengths of the three sides of the triangle are $7,8+4=12$ and $2(8)+1=17$, of which the longest is 17 .

ANSWER: (C)