Question 801291
You need more information than just the perimeter. The information may be in a drawing that comes with the words of the problem.
 
If you have expressions for the length of each side, such as {{{x}}}, {{{x-3}}} and {{{2x+5}}} for example, you can write an equation that would look like
{{{x+(x-3)+(2x+5)=perimeter}}} --> {{{x+x-3+2x+5=perimeter}}} --> {{{4x+2=perimeter}}} --> {{{4x=perimeter-2}}} --> {{{x=(perimeter-2)/4}}}
 
If the additional information was the measures of angles, it would be a more difficult problem, but I am guessing that is not your case.