Question 163826
You're getting there but you only used half of the information.
"four times its lenght is twelve less than its perimeter"
{{{4L=P-12}}}
{{{4L+12=P}}}
Now you have another equation for P.
Put that together with your other equation for P. 
{{{P=2L+2(2L+3)}}}
Equate them and solve for L.
 {{{4L+12=2L+2(2L+3)}}}
Re-post a question if you get stuck.