well, we know it's a regular polygon of 5 sides, namely a regular PENTAgon, and since we know one side's length, the perimeter is simply that much 5 times, namely
[tex]\begin{array}{ccllll} &2x^3-5x+3\\ &2x^3-5x+3\\ &2x^3-5x+3\\ &2x^3-5x+3\\ +&2x^3-5x+3\\\cline{1-2} &\underset{\textit{\LARGE Perimeter}}{10x^3-25x+15} \end{array}[/tex]
