Question 1116264
You really need first to find the pair of slanted segments on the left and right sides.  Your figure shows you some distances to use which form a right triangle.


Either left or right side segment, hypotenuse of a right triangle.
{{{h^2=8^2+16^2}}}
{{{h=sqrt(320)}}}
{{{h=8sqrt(5)}}}
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Question was for finding perimeter.
{{{2*18+2*h}}} and you know h.