Answer
A rectangle of length $3x$ and width $2$.
Work Step by Step
The triangle is made of the sides with lengths $x+3$, $3x$ and $2x+1$. The perimeter of a triangle is the sum of all sides' lengths. Adding the sides' lengths, perimeter, $P$, will be $$P = x + 3 + 3x + 2x + 1.$$ Grouping variables and constants we get: $$P = 6x + 4.$$ Let's construct a rectangle of length $l$ and width $w$ with the same perimeter $P$. The perimeter of this rectangle is obtained by adding the $2$ lengths and the $2$ widths:
$$\begin{align}
P& = l + w + l + w\\
P& = 2l + 2w.
\end{align}$$ Since this perimeter should be equal to the perimeter of the triangle, we must have $$2l + 2w = 6x + 4.$$ Dividing both sides by $2$ we get: $$l + w = 3x + 2.$$ For example, we can take $l=3x$ and $w=2$, thereby resulting in the rectangle shown in the image.
