Question 94620
You have all the information here, just solve for L using the equation you provided. This is the standard equation for perimeter of a rectangle.
{{{P=2L+2W}}}
W=9
P=26
{{{26=2L+2(9)}}}
{{{26=2L+18}}}
{{{8=2L}}}
{{{4=L}}}

Your Length is 4ft.

Check...
{{{26=2(4)+2(9)}}}
{{{26=8+18}}}
{{{26=26}}}